Originally Posted by charliBum
and can some one please tell me how a hunch back can badly effect your jumping position?
You should have an open chest and your shoulders back. If you have your shoulders back and open it helps to prevent you from collapsing forward and helps to center the weight of the upper body over the lower leg, helping to keep you be secure in the tack.
If you do not have a stable upper body, you will not have a stable lower leg causing an unsafe position.