As Montanan already said, hang the bag. Do not lean it up against a board, it must be able to move a little to help absorbe the arrow.
Example.
1. Take a 1x4 and nail it between two post. Box it as hard as you can and the board will break.
2. Next take a 1x4 and hang it from a string and again box it as hard as you can and youll find you cant break it..
I'm not actually trying to get you to break boards or your hand, but I think you'll get the point.
Try splitting wood with an axe while the wood is hanging in the air with out something solid, you can't do it
Oh...Never mind thats dangerous too, JUST HANG YOUR BAG if you feel better with wood behind it then put it about a foot behind it and the bag will stop your arrows.