I just copied and pasted what I posted on that thread.....hete it is...
This is very typical of a new shooter just starting out. Hopefully this answer will get you started off correctly.
If you DON'T point your fingers back toward you....when the draw gets heavier, you are curling the fingers back toward you to help you with the weight.....and YOU are rolling the arrow off the shelf.
If you WILL point the fingers back at you to begin with, the weight will make the fingers UNCURL ...keeping the arrow ON the shelf.
I covered all this and reverse cant on The Bowhunters of TradGang DVD's shooting section.