There’s two angles to the fist the inside of the middle knuckle(two knuckles) or the outside of the middle knuckle(three knuckles) its much more natural to hit with the outside of the knuckles but it causes more rotation and less power . One interesting point is you can only hit with the outside knuckle in modern boxing gloves where the inside of the middle knuckle really lines up the arm wrist and fist kinetically and is a step up in power, but also vastly reduces the chance of a boxers fracture because you’re not throwing pinkie finger first.
Here’s one of my teachers is 5 dun Kyokushinkai, he worked 27 years as a doorman and never hurt his hand hitting a person, he hits with the first two knuckles (or inside of the middle knuckle to be precise), with a slight bend in his wrist.he also using the deep and the ripping shockwave here’s a clip of him training for power on the bag, because he’s training for door work his hands aren’t up in a guard all the time, the bag work starts 40 secs in.
The biggest problem he would have after a session is the shock to the forearms, because the body is lined up perfectly kinetically for a bare knuckle punch. the wrist is held on a slight bend which is its strongest position, whichever knuckle you land,the weakest point is the kinetic chain here is the forearm about 3 inches up the arm from the wrist,when you hit with this power you end up flexing your forearm bones,his forearms would ache for weeks after a session,as well as with the punching, I can also feel this forearm flex from a good hooking palm heel on a very heavy bag
I noticed a significant different in power, hitting with two knuckles compared with three knuckles, its also worth noting you can't land a proper two knuckle punch in boxing gloves because of the fixed thumb position, if you hold your hand in the same position as it is in a boxing glove, then try to land a karate style two knuckle punch slowly on a wall, you can see that the thumb hits first, you can’t move this in a boxing glove.so big glove punchers tend to favour a the weaker three knuckle punching style.
Here’s two pictures the first one shows my fist landing with the outside of the middle knuckle(three knuckles) you can see how the elbow is down and you lose allot of kinetic power here, because the arms its not behind the strike supporting it, there's less drive and this causes the fist to spin off quicker with less power delivered and less shockwave on the end
in the second picture (here you can also see the weak point on my forearm) I'm landing with the inside of the middle knuckle(two knuckles) the arm is right behind the strike to drive in the kinetic power in this create significantly more shockwave, for this reason I find I can punch harder bare knuckle or with mma gloves using two knuckles,than I can in boxing gloves and three knuckles.the dvd is out now links further up on the blog