When backchecking against a 2 on 1 it is key for the defense to recognize the situationa and communicate with the foward. The defense should take away space from the puck carrier when they realize that the backchecker can get to the other player on the weak side. Here are some key points:
defense recognizes they have a backchecker
defense comunicates with backchecker to take the weak side
defense takes away space from puck carrier and makes it tough for them to pass the puck to the middle of the ice with good stick position
backchecker has to move their feet until they are on defensive side of the weak side attacker
backchecker needs to take care of the attackers stick so they cannot get a pass or a rebound