I was always taught that the hand slips in from the bottom of the strap, and then grasps the strap and the grip. This allows for support of the hand with the strap and at the same time if you let go lets the hand can be pulled thru. Secondly if you have the pole yanked out, thumb tucks in and poles slips off.
Been skiing that way for over 40 years and have yet to have skiers thumb. Bent bones, cut face (safety strap days), cracked vertebrae (oops, that one hurt, still does occasionally, play hard, pay hard), tweaked elbows, shoulders, knees, ankles (falls), but never a thumb.