To fool someone in thinking that what they thought of, said, or did was cool. Enticing them with a high five, than retracting said high five.
After said retraction of sarcastic high five with accompanying "PSYCH!" it is customary to say, "I just high five psyched you!"
twitard: Dude, I totally saw that new twilight movie! it was awesome!
Smart Guy V: Really? Cool! High five! (raises hand to initiate high five) ... PSYCH! Twilight sucks! I just high five psyched you!