In the defensive zone, the defensemen must work cohesively with the center-man, communicating constantly to make sure the attacking forwards are accounted for at all times. Although the center and both defensemen will rotate positions frequently in order to maintain good defensive coverage as plays develop, the left defenseman will generally be responsible for the left half of the defensive zone from the half boards to the low slot to the boards behind the goal line and the right defenseman will be responsible for the opposite half.
Both the left and right defensemen and the center-man are responsible for keeping the crease clear so the goaltender is able to see the puck at all times. The attacking forwards will try to distract and interfere with the goaltender in order to keep him from being able to play the puck effectively, so the defensemen must patrol the front of the net and protect their goalie from the opposing teams' forwards to give him an opportunity to stop the puck.
It's important for defensemen to force the attacking forwards to the perimeter of the zone where they are unable to take shots from high-percentage scoring areas. Over-committing to an attacking forward can put a defenseman in a compromising position. The best defensemen have excellent gap control, which means that they are able to keep themselves close enough to an opposing player to neutralize them without getting so close that they can be burned without having space to recover. Always stay between your man and the net and keep him within 1 to 2 sticks' length.
The #1 rule for defensemen in the defensive zone: never pass the puck up the middle. Move the puck up the boards to a winger, to the swinging center-man, carry it out yourself or simply flip the puck up and out off of the glass. It's better to ice the puck than to force a pass up the middle of the defensive zone.