In a man-to-man defence, the marker is responsible for preventing the thrower from throwing the disc to a large area of the field. Australian Ultimate: Marking
Marking is an active process, not a passive one. It involves reacting to whatever the thrower is doing to try to make any throw a more difficult one. The harder the marker works, and the better the mark, the less work defenders have to do to shut down their receivers.
So how do you put a good mark on a thrower? The following are general tips to improve your marking if you are not already doing them :
In a man-to-man defence, the marker is responsible for preventing the thrower from throwing the disc to a large area of the field. The team should agree in advance on a significant object or objects on each side of the field, say a clubhouse on one side and trees on the other, or use generic terms such as "Home" and "Away" which can apply to any field. The marker then calls a force, eg "Force Home", which means that he will only let the thrower throw to that side of the field. He does this by standing at about 45° to the thrower on the opposite side to the force. The defenders then assume that throws are far more likely to come to the force side, and defend accordingly.
It is the marker's responsibility not to let the thrower throw in the opposite direction to the force. If this happens, it is called a break of force, and often results in the opposition getting two or three easy passes in a row, as the defenders are assuming that the force will be held. If a break happens, the marker should call "Break" loudly and clearly for the benefit of the defenders.