The following is taken directly from Bowlingball.com. It is a good explanation of axis tilt. It also provides a clue (the position of your thumb at release) on how to alter your axis tilt.
"Axis tilt is the vertical angle at which the ball rotates. Axis tilt is what is commonly known as spin. Axis tilt is determined by the position of the thumb during the release. If the hand turns too early, the thumb exits on top of the ball. Bowlers with a high degree of axis tilt will be able to see the top of their hand during the release and follow through. Being able to have the thumb exit at the bottom of the down swing minimizes axis tilt.
Axis tilt will induce skid and reduce the amount of backend hook potential. With the core rotating in a more vertical fashion, oily lanes will be the enemy, drier lanes its friend."
--------------------
Gary Palma
Owner of X-act Reaction pro shop at Ideal Lanes
www.myideallanes.comPBA Member