While I agree the simplest advice is to bend at the waist.....Bending at the waist is a way to keep you from creeping or shortdrawing. Lowering the bow arm in effect shortens the draw length as it impossible to not "give" somewhere. If you don't give at the waist you give in the shoulders.

Just always was one that wanted to know "well, why?" Yes I was one of those....LOL! So, I like to teach at the "why" level.