A "low five" invented by a Cannabis connoisseur known as "Charlie" whereas the downward "five" is executed with then retracted as quickly possible with a severe excess of the force expected when receiving a "high five".
(It can also be used in a sideways variation)
This is the exact opposite of the 'Charlie Tap'
You know how I can tell he's excited about tonight? He gave you the 'Charlie Slap'...