In Ultimate Frisbee, the force is the direction the defensive team attempts to force the offensive team to throw to. The basic idea is that the mark stays on one side of the player with the disc, making it impossible, or at least harder, to throw to that side of the field. Then the rest of the defenders don't have to worry about covering their men as much on that side of the field, and can stand slightly to the strong side of their men. This looks like this:

Capital O has the disc, o is offense, d is defense.


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  |		Od	do-dump				|
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  |	strong side	d  o		weak side	|
  |			d  o				|
  |			d  o				|
  |			d  o				|
  |			d  o				|
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Notice how the defenders are standing to the left of the offense. This means that any offensive player who cuts to the strong side is going to have to beat a defender who has a couple of steps on him already. On the weak side, the offensive player has a couple of steps, but it is much harder to throw to that side. The force is a pretty basic concept in frisbee defense. If you screw it up while you're on the field, people will scream "WRONG FORCE!!! WRONG FORCE!!!!" at you to remind you that you should be standing on the other side of your man. The force can be toward either sideline, and some teams will force middle, meaning that the mark always stands with his back to the sideline, trying to force the offense to throw towards the middle of the field. Sometims you will want to force straight up, or stand directly in front of the player you are marking in order to deny them the huck throw.