I want to wrap up my general discussion of defensive positioning with some comments on how to play defense off-man. By off-man, I mean with a large cushion of 4 or more steps. I am not talking about scared or lazy defense where you back your player to avoid being beat deep, but a hungry aggressive defense that is looking to cover one-and-a-half. (One-and-a-half meaning your player and help on others.)
1. The foundation is anticipation and energy. You anticipate the action and use energy to bring yourself to the play. You must constantly watch the run of play looking for openings. You recognize the space when your offensive player does and break into it simultaneously.
2. You need protection to play off. Because you are allowing separation, you are vulnerable to a one-pass shot to your player. That's why playing off of the front player in a vertical stack is usually pretty dumb - the thrower just beats you. There are different kinds of protection but most fit into three categories: traffic, distance and value. The classic off-man position is last back. This works because there is usually a lot of traffic underneath that prevents the thrower from hitting your player with one throw. (Incidentally, this defense was a big motivator for the expansion of the flat stack which moves the traffic out of the way.) Distance is another big help. If your player is far from the thrower, it is easier to play off. The throw is tougher and is in the air longer, giving more time for recovery. An example of this is when you are covering a sideline cutter in a flat stack and the disc swings away from you to the far sideline. That shot, some 50 yards across and up the field, is ridiculously hard and allows you the luxury of dropping off of your player and helping to the middle of the field. The last kind of protection is value. Teams poach off of the swings in horizontal offense all the time because the swing to the sideline has little value to the offense.
3. If you are going to play off, be prepared to switch. You can see the whole field. There will come an situation where you or a teammate is beaten and need to help each other out. The key is to close out the separation immediately after the switch. That is the energy piece. Anticipation sees and makes the switch, energy closes it out so that you don't give up an easy shot.
Done properly, playing off-man is very effective, but it really requires smart play and a lot of mental work. The advantage is less physical work and an increased likelihood of a help d. The disadvantage is that when it doesn't work, it looks really, really bad.