you should only punch with the first and second knuckle (index finger and 2nd finger)...

this is for transferal of power.

hold your arm out straight, draw an imaginary line from your forearm right down to your fist, the straightest line is to those two knuckles. This is what you should be using when you punch as they have the full support of your arm behind them where as the 3rd and 4th (pinky) are off to the side and little support.

if you swipe at someone and hit with the 3rd and 4th knuckle you will know about it quickly :P

i have to punch through wood in martial arts and its easy when you know the body's mechanics,