It starts out like a high five, you raise your hand and when the other person raises their hand, start to do a normal high five but right before your hands touch, bring down your hand, rotate your hand 90° to the right and tilt it forward so it should just be your hand is horizontal with the palm facing down, dodge their high five, and karate chop them in either the chest or stomach. For added effect, say "High Five!" before and then say "Betrayal Five!" after you hit them.
Shawn: High-five, dude!
^Instead of a high five, Shawn slaps Albert in the chest area^