Remember : the correct way is to chop (or more likely roll) through the knuckles. Align the thumb joint out and higher, just before you start.
The last point makes more sense when thinking of doing backhand undercut chop or roll, in squash. You learn fast to get your thumb joint out of the way (since otherwise racquet handle following through with the under-roll hits it, with dislocation a possibility) . Forces in squash are bigger than their micro versions used in virtuoso piano play.