I am a rather poor slapper. I tried the thumb up technique for awhile, but couldn't get it to work for me. Bad tone, and no precision. I've settled on the thumb down approach which allowed me to get decent tone and accuracy fairly quickly, but I now realize that I can't double thump using that hand position. What I'm wondering is how the shape and flexibility of your thumb and its joints affects your slap technique. When I hyperextend my thumb, it probably is only 10 degrees off straight. A lot of the slappers using thumb up technique seem to have thumbs that bend backward at 30, 45, or more degrees. Am I just making excuses, or have you also noticed that thumb uppers tend to have 'doublejointed' thumbs, while thumb downers have thumbs won't bend backward very far?