Scoring a backdoor play against an overplaying defense is one of the best feelings basketball has to offer. The cutter has to get far enough out and wide enough to make the pass easy to throw. If the cutter stays in tight, the pass has to be perfect to get through.
Good players should already understand that. What good players, however, often fail to do is communicate with each other so that potential backdoor passes don’t go sailing out of bounds because no one has cut for them.
The way to make sure this never happens is to make a rule: If the cutter takes two steps, that means he is going all the way to the basket. He can jockey with his defender and move his shoulders back and forth and try to duck behind the defender, but the passer should not have to guess whether or not the cutter is going. As soon as the second step begins, the cutter is going. If both the cutter and passer are sure of that, the cutter can be led and can score even when he is very well guarded.
Sometimes, a coach will make a rule that a player may not come farther out than a certain point to get the ball, so when he gets to that point, both the passer and he know that he must go backdoor. If the coach sets up backdoors against pressure in a certain way, terrific. But if he doesn’t give you a specific rule, this two-step rule will be helpful.
From Dick’s book Stuff
Welcome To Keys To The Gym 
Another opportunity for a backdoor cut in addition to a defender overplaying the passing lane is when your defender turns his head away from you to locate the ball. In that split-second, you usually have a golden opportunity to make a cut — usually a backdoor cut.
Most players only watch the ball on offense and miss these cutting opportunities. They would do well to closely watch the head position of their defender as much as they watch the ball.