Not sex between hands and genitals (
fingering or
hand jobs); sex between your hand and the hand of another person (or between your own two hands, but that doesn't really work for basically the same reason
why tickling yourself doesn't work). It may sound silly, but it really works. Though I've never actually had an
orgasm from hand sex, I often get really aroused and have one hell of an
erection. And it's
fun in itself.
There are probably as many different techniques and positions for hand sex as there are for conventional sex. There's one in which only the fingertips touch, one where the fingers of each hand curl around the thumb of the other (hand 69?), and of course the standard interlocking fingers position. There is heterosexual and homosexual hand sex (which aren't really that different), group hand sex (which works best in a circle), and all sorts of kinky hand sex.
Most people seem oddly reluctant to take part in hand sex when it is explicitly labled as such, even if they don't have a problem doing exactly the same thing under a different name.