If you do them for punching, first two knuckles. If you do them for extra few inches of range of motion, on the whole fist.
I prefer the first two knuckles, palms facing inwards, elbows tight. Of all the variations I've tried, these seem to work the best for me. No, you don't have to do them on a hard surface. Most of the time I crank these out right when I wake up on a thin carpet. Most of my hand conditioning is done by other means, but when I do them for hand conditioning I prefer my sidewalk.
