Let me preface this entry with the following: this is not HOW to flat mark. This is when and why you should flat mark. If you want tips on how to flat mark, or what you need to do to have an effective flat mark then you'll have to find them elsewhere, or wait for a later entry, because tonight we're talking flat mark strategy.
A lot of people think that a flat mark is only useful against hucking, which it certainly is, but strategically the flat mark is one of the strongest all-around ways for a team to mark, regardless of what the opposing team is trying to do.
For starters, I want to be clear about terminology and the absolute basics. With a flat mark, instead of forcing to one side of the field, the marker is standing directly in front of the thrower, effectively eliminating the inside area of the field behind them in about a 45 degree arc. On top of that, there are two types of flat marks: the traditional flat mark (no-huck) and what I call the strike mark.
A traditional flat mark is designed to make it difficult for the thrower to throw the disc deep. To flat mark effectively there are two things that come into play.
First and foremost, you're eliminating both the inside flick and backhand. The best hucks are inside-out throws that travel flat and allow for an easy read and catch for the reciever. As a flat marker you are attempting to eliminate these. If they get any kind of throw off, deep or otherwise, you're forcing them to throw it around the outside of your mark, resulting in an outside-in throw.
The second part of an effective no-huck flat mark is being aggressive as a marker and making it very difficult for the thrower to step forward into their throw. As they pivot from forehand to backhand you should be shuffling laterally and your body should be impeding their ability to step into a throw. There are few throwers (although they do exist) that can step back and around a mark and still put off a 60+ yard huck. And for you rules zealots out there, I'm not saying you should be violating disc space or intentionally fouling them as they pivot. Your job as a marker is to anticipate and move with them, they can't step into a space already occupied by you (that's a foul on them). Force them to step backwards or straight out, off-balance throws are hard.
The second breed of flat mark is what I call the no-strike mark. This is a mark that is completely designed to eliminate the inside-out throws (essentially shutting down a strike cut, a cut coming into the inside throwing lanes). The setup for a no-strike mark is the same, but you're playing off on the mark several feet. The farther you back off, the more you eliminate both the forehand and backhand inside-out angles. You're allowing them to stop into their throw and pivot uncontested, but you're making the inside-out throw impossible. The only choices for them at this point to get a disc into the strike zone is to either throw a high release over the top of you (which is easy to handblock the farther off you are), or to throw something VERY outside-in that will bend around the outside of your mark, which is a low percentage throw or should be shut down by your downfield defender.
As for when and why you should flat mark there are three rules of thumb that I use.
1) When the opposing team is going upwind. I absolutely hate playing zone in an upwind situation (you're allowing their best throwers to throw their strongest throw the majority of the time), but if you flat mark effectively, you're going to severely limit what their offense can do. To throw in a stiff breeze the disc needs to be thrown inside-out to get any range, and if you can eliminate that with your mark then their offense will be severely limited, because the areas that the disc can be successfully thrown to (close range underneath throws) are easily coverable by your downfield defense.
2) They are beating you with a lot of line (oven) cuts from their handlers. If you're facing a team with a lot of quickness in their handler core, and you are consistently beat up the line, a flat mark can be very helpful. Essentially, you're making the line throw a break throw (albeit very slight) regardless of where the disc is on the field. While a 4-5 yard inside out flick or backhand isn't the most difficult throw, it is still more difficult then an open-side uncontested throw. With some communication on strike calls and line calls from your sidelines, you can have the mark be shifting the shade of their flat mark (from forehand to backhand) and you can make that break even more difficult.
3) They have one or two dominant huckers. This one is simple. Take someone with a good mark and try and shut down their deep game. I think back to a game we played against Pitt at Terminus in 2005. They beat us on Saturday mostly through a deep game that revolved around one lanky thrower. Sunday we faced them again and put our longest athlete on him flat marking the whole game, victory.
Obviously there are downsides to flat marking. The downfield defense can't cover both sides of the field so you're conceding a certain amount of underneath game. Flat-marking is most effective against offenses that are trying to work the disc through the middle of the field (split, spread, etc). Center stack, while a very huck oriented offense, is difficult to flat mark against, because the outsides of the field are always left very open.
The end all moral of the story is this: flat marking has its place in a lot of different scenarios, not just against the huck, try it out.