Remember while a relaxed swing is optimal, you still want to lift at your ankle. If you're completely relaxed all the way through, the ball is just rolling straight off your hand and you're coming directly up the back of the ball, which is my first guess as to why you're thumping. Lift at the ankle and follow through. If that doesn't seem to change anything, then I would take a closer look at your thumbhole and make sure you're getting your thumb out before your fingers, not at the same time. Your arm is relaxed, but how tighlty are you holding or squeezing with the 3 fingers in the ball?
