JGW, I really don't want to hit anyone with the "side" of my metacarpals, or any-thing, for that matter. Owwwww... I have to use my hands to make my living! I know, I "could" do some really neat-O hot sand in a pot training from the old Kung Fu Theatre flicks (really, that's what it was called. Sunday night, on Channel 8 in mid-Missouri) stuff to get my hands tougher... but I really don't have the time, or inclination, anymore. I didn't even do the spinning backfist with that part of my hand, mine was a spinning hammer fist. It was functional. Worked OK. Let's see who "thinks" about their backfist the way I personally do. Right this second I'm considering being right-foot forward, typical hands-up fighting stance of whatever variety you prefer, right hand slightly closer to opponent than the left. The below considers my right hand firing with the strike. When I jab, typical western-boxing style jab, my shoulder rotates counter-clockwise (to my perception, not my opponent's) as the arm extends, the elbow correspondingly rotates in space counter-clockwise, the right shoulder itself rotates counterclockwise as well a bit and moves towards the opponent, and at the terminus of the strike - I'm focusing on 1st two knuckles of the right hand's fist, those two knuckles are also rotating counter-clockwise although I'm picturing them travelling down a straight line into the target.... as the match/fight goes on and fatigue develops, the line generally degrades into a more looping thing, but it still "looks" like a jab. Body slips forward slightly, hip rotation all the standard body movement and mechanic things to develop the power with which the strike connects. Backfist... is the same "body" things, foot slide, hip rotation, shoulder roll forward... but all of the mechanics in the arm itself are ... inverted (I'm not certain that's the right word). The shoulder socket itself rotates clockwise a bit, the elbow follows the clockwise rotation, and at the moment of impact the 1st two knuckles, though travelling in a straight line (I hope) are rotating clockwise. As TKD warriors are occasionally wont to do, we tested the "strength" of each other's strikes with... of course, board breaks! To this day, I have no idea what that pine tree ever did to us, that so many TKD people just had it out for it... Anyway, back before I gave up breaking inanimate things, I could break 2 1' sq. 1' thick boards with the backfist, an 3 with the jab. The backfist was quicker to get on target, the jab brought more with it and the internal body torque set up different lower body combos.