When someone offers up a high
five and you give them the back of your hand instead of the
palm. Could also be described as a "half a
five."
A team scores and some dude says "high five man!" then as you are giving the high five turn your hand around to give the "back five."
Maybe because you want to be cool with that
person (not leave them hanging) but you'
re just not into it - it's like a weak high-five or a half-high-five.