In man-to-man coverage, the most important person is the person putting on the mark. Basically, he is covering half of the field, with his six team mates responsible for the other half. If he gets broken, then the defence as a whole is pointless. Being broken means the disc being thrown to the closed side.
To avoid this, it is important to mark with the hand closest to the stack down low and the other hand up high. Also, it is important to keep on your toes and move around a little bit, to avoid players leaning back and breaking you. One of the most common mistakes is when the marker gets a bit too anxious, and tries to block a throw to the open side. If the thrower fakes and quickly pivots, he very often has an easy break throw.
There are six other defenders who have various responsibilities. The person marking the dump pass should mark as enthusiastically as any other defender. Very often, defenders consider it a success if the team has to dump, but a good team will take advantage of quick give and goes which allows them to keep the disc moving and the defenders can’t get a stall count going. This puts no pressure on the throwers and allows them to choose the best cuts to pick out.
As for the 5 defenders marking the people in the stack, the first four should stand a little in front of their opponent, on the open side to give them a head start when the offence cuts. The person marking the deepest player should stay behind their man, and help out if he sees anybody going long unmarked. When you are following a person to the mark, it is important not to make too much of an effort if you don’t think you are going to make it. If you are too enthusiastic, then you can often go past the man and allow break side throws.