The act of a regular fist bump, yet the fist is upside-down. This kind of fist bump is a rejection of the purpose of the fist bump, yet must be done for the purpose to fit in.
Ted: Dude, I just totally busted that wimp in the face
Victim's Friend: Oh, yeah man
Ted: (Requests a fist bump)
Victim's Friend: (Uses an Upside-Down Fist Bump)