ORIGINAL: skeeter 7MM
Try this to get the feel, draw your bow straight then acquire the target below without moving your arms. Only one way to do this and it is bending at the waist!
By dropping your arm to acquire the target below you in fact change your draw length, why when doing so your POI shifts up. If you bend at the waist there will be no difference in POI from an elavated position vs ground.
There are 2 reasons why point of impact change when shooting from an elevated stand, one would be not bending at the waist because your anchor point has changed but the other is gravity. When you shoot your bow at 30 yds from the ground gravity is pulling it down so you set your pins to accomadate for this, now when you go up a tree or stand and use your 30 yd pin that was set on the ground and shoot at 30 yds your POI will be high no matter if you bend at the waist, because gravity is not pulling the arrow down because you are shooting down, thats why you will hit high. Same thing with a rifle that has been sighted for say 200 yds, shoot down a mountain at something out at 200 yds and it will hit high also.