OK, I'm finally done with the anecdotal entries. It's time for the meat and potatoes: the strategy! If you haven't caught up with Part 1 or Part 2 please do so.
Part 1 - When?
Let me preface this by saying that what I'm outlining below is assuming a certain level of play. If you are still playing in league/college/club where people are consistently struggling with basic forehands, then this is all moot; force them forehand!
The game of Ultimate has changed significantly in the last ten years. People like to argue this player from this era versus that player and whatnot, but let me say what I believe has changed: the overall depth of talent in the sport. I don't believe today's top players are better than the top players ten or fifteen years ago, but middle-of-the-pack players are unbelievably more skilled than when I began playing. Ultimate players, in general, are starting the sport earlier, dedicating more time to developing their skills and I think overall the depth of play is way beyond what it was in the early 2000s.
This is partly why teams I'm in charge of have moved away from a traditional force all the time. Every single decent team has strong throwers who can huck, break the mark and consistently make tougher passes. Down-field defenders simply cannot cover enough space because too many players have strong throws and more space on the field is threatening than ever before as a result.
Part 2 - The Challenges
Let's break down traditional man defense with a force: why is it difficult?
1) The offense always has the advantage (this is true for any kind of defense).
The offensive dictates what is happening on the field and the defender is forced to react. Good defenders anticipate, dictate and force cutters into uncomfortable situations. However, the nature of the game, as a non-contact sport, gives a huge advantage to the offense. Good shut-down defenders tend to be players that are simply better athletes than the cutter they are covering; they are able to shut their man down or get a block simply because they're quicker, faster or can jump higher. However, even the absolute best athletes in the sport still get beat on defense because of this offensive advantage.
2) Down-field defenders are given the difficult task of covering both the underneath cuts and deep cuts.
I can confidently say I can take away an force-side underneath consistently, even without being an elite athlete, but that leaves the deep open and vice-versa. Some of this can be mitigated with good help defense, but without a significant athletic advantage, a defender simply cannot shut down the entire force side of the field by themselves against a decent cutter.
3) Dump defenders are given the difficult task (depending on setup) of covering both the line and backfield cut.
To shut a cutter down 100% as a dump defender, you have to prevent both a line cut and a backfield cut, which is challenging without a significant quickness advantage over the person you're covering.
4) The throw-and-go.
If you are covering the thrower, you are positioned on the wrong side of the after they release the disc. Not every offender will take advantage of this, but they absolutely should. After a throw-and-go the defender has to make up ground and reestablish their force-side presence, which can be extremely difficult against a team that moves the disc quickly. Teams without a lot of depth can base their whole offense around one player who can throw-and-go and get the disc back after every throw.
5) Defense is grueling in Ultimate.
There are few sports where the competition structure (tournaments) and game structure (everyone playing both offense and defense), has the players running as much as Ultimate. Yes, fitness is 100% in your control and you never want to be the guy being "out-ran" by the other team, but the fact is that it is next to impossible go 100% every point. Inevitably players concede a handful of easy catch-and-throws simply because their tanks are running low.
Part 3 - Overcoming The Challenges
First and foremost, this defensive structure (or any) does not work without confident, consistent marking. Yes, a good thrower can always break a good mark, but unless your marks can consistently offer up some disruption to the thrower with shifting, employing the marking triangle and understanding throwing angles, then this defense (or any) will fail. If your team hasn't spent significant time on core marking principles, then stop reading right now and go do shuffles for the next six weeks. Defenders have to be able to to trust their marks. Assuming a baseline of decent marking, here's the strategy breakdown and why:
Don't pick a force on the line. As the field shortens and there is less and less deep space, a traditional force becomes easier to maintain because down-field defenders can commit more to the underneath cut. However, off the pull, rather than a forehand or backhand force trying using this priority order for a force communicated on the field in real-time and on the sidelines:
a) Back towards the direction the previous throw came from
b) Towards the wider side (where there is more space) of the field horizontally.
c) Towards the side of the field the offense has their dump set up on.
Why back in the direction the previous throw came from?
First of all it helps to mitigate the defensive disadvantage of the throw-and-go. If the marker knows the next marker will send the disc back in the direction it came from, then the original marker is already on the correct side of the disc when thrower takes off. You don't have to overextend yourself trying to re-establish position which can oftentimes lead to being beat back to the force-side anyway.
Secondly, a lot of offensive structure is based on the idea of attacking a specific side of the field with a succession of throws. Vertical stacks in particular tend to generate offense from several breakmark throws in a row. If the breakmark side of the field changes every other throw, this significantly disrupts down-field continuation. It may not stop it entirely, but it still disrupts the desired flow.
Why towards the wider side of the field?
This is firstly to help the down-field defenders with covering their deeps. This tactic is all about taking advantage of poor deep spacing that comes with the deep cut originating from the force-side of the field.
The easiest deep looks are generated from the break side of the disc. The less and less break side of the field there actually is, the harder it is to get a space throw off and the more the mark can flatten out.
Think about a disc trapped all the way to one side of the field. The benefit of a trap, in theory, is that a thrower has next to no force-side field space to throw the disc. However, the entire field is now break space that the marker needs to cover and any force-side deep throw that comes off will be be to a cutter with good horizontal deep spacing (aka a throw that's easy to get good distance on while still being easy to read)
Now leave the disc on the same sideline but flip the mark around, forcing the disc back to the wide-side of the field. The entire field is now 'force-side' space technically, but it makes both down-field defense and handler defense easier.
The mark can now flatten out completely. Break-space is covered by the sideline. Instead of the mark feeling like they need to take everything away (as with a trap), they can stay flat and shift liberally to cover inside-out throws.
Down-field cutters can feel very safe taking away the underneath cut because a deep throw from this spot either has to come off inside-out against a flat mark or has to bend around the width of the field, which, even for an above-average thrower, is significantly more difficult and will not travel as far. These throws tend to float and give even slower defenders a good opportunity to make up ground and get a D.
For handler defenders in this situation you know that the up-the-line space is covered by the mark and the sideline. So instead of worrying about taking away the up-the-line as you would with a trap you are free to position yourself on the backfield threat, trusting your mark to contain the line.
Certainly there are still throwers in this situation that can break the mark for the up-the-line throw but just as with the deep throw from this spot, it is more challenging.
As the disc moves further and further away from the sideline the mark can't stay as flat (the sideline covers less break space the further you are from it) and the down-field defense has to feel more and more threatened by good deep shots coming off. Adjustments in positioning will need to be made.
In general though, the more open field there is on the force side of the disc the more the defense can cover the underneath cuts from the down-field and the backfield cuts from handlers, aka the meat and potatoes of most offenses.
Why force the disc towards their dumps?
As third in the priority chain, forcing the disc towards the dump(s) is pretty straightforward. Dump defense is easier on the force side of the disc because the throw up the line is a break throw. As outlined above the closer and closer the disc gets to the sideline, the flatter the mark can and the more the dump defender can focus on taking away the backfield reset.
The other advantage you get with forcing a disc towards the dumps is the handler defender can sag into the throwing lane (or lanes against 3 handler sets). From the middle of the field this will force the disc closer to one sideline or the other (assuming the thrower hits their poach) where then establishing the best mark (as outlined in the previous section) becomes more obvious.
Part 4 - What are the main problems?
Part of what makes this defense challenging is it's dependent on every one of the seven defenders on the field playing the same way. A mark the wrong direction or a loose defender can break the defense wide open very quickly. It takes a lot of practice to execute well, and when it's not going well it looks terrible.
Mistakes are also punished more heavily. One leaky mark or a good bump back to a thrower can break down the whole defense. When the downfield is counting on well-spaced hucks being shut down, they are significantly out of position if they do come off.
To combat this, there has to be a shift in defensive mentality for a lot of people. The top priority for a defender has to be that around contain (sending the disc back towards the previous throw). This means that defenders have to stay in control of their body. They can't over commit to getting a big D. They can't jump the inside lane of throw unless they're confident in getting that D because around-contain will be impossible afterwards.
Recognition of when your tight defense has to loosen up for the containment of the next throw is absolutely paramount, and what younger players struggle the most with. You can't take the shortest path to the disc, you have to understand where your next threat is; and will an ever-changing force, the threat will always be the around.
Overall, it's difficult to execute well, but when it's clicking, it can be unbelievably frustrating to play against and can take even a disciplined offense out of their comfort zones.
I apologize for the delay in posting this entry (I was waiting for my team to finish up at Pres Day) and I apologize for the lack of diagrams (I'm terrible at them). I'd be happy to answer any questions to alleviate any confusion. Yes, this is really just a slightly more in depth force-middle.