A lot of posters are talking about "leaning" backwards. It's not so much the leaning that I'm concerned with but the "falling back" which prevents displacement contact from being made. If a player is leaning back and contact is made it's pretty easy. If a player is falling back it typically causes the offensive player to fall on top of the defender, whether his momentum takes him there or he trips on the defender. Either way it's a block for me.