When you are about to give someone a high five, right before the hands touch, you turn the wrist of the raised arm 90 degrees, mimicking an air vent opening up. This causes the recipient of the prank to miss the high five and stumble forward under his own momentum as you walk away proud.
For the two handed high five, the double air vent typically results in a hug.
guy- "watch out this guy high fives really hard dude"
you- "ok, noted. check this out"
(successful air vent. kid on ground)
guy- "wow, i've never seen anyone trip up so hard! You vented him like a champ."