Made famous by Special Agent Leroy Jethro Gibbs (Mark Harmon) on the t.v. show NCIS, this is an open-handed slap to the back of the head. This is usually done in response to an act of stupidity. Done in a half-serious way to show power.
Once at work, I was playing Tetris on my computer. When the boss saw, he Gibbs slapped the hell outta me!