Someone once told me to shoot just as the deer begins to lift it's leg to take a step. Do not shoot while all four feet are on the ground. Seems that the act of moving in a deliberate way adds some delay to their reflexes. Sounds good in theory. I have never used it in an actual situation. Aiming low could backfire. If the deer does not react., you could wound it or shoot under. I have seen or heard of a great many that do not react at all.