What is bouncing someone's forehead called

Bouncing is a recreational activity that usually involves gently tapping one's own or another person's forehead with a finger or other object, and then tilting the head back so that the tapping part bounces upward, similar to a bouncing motion. Thus bouncing someone's forehead is called bouncing the brain or hitting the forehead.

The name bouncing or smacking is a more intuitive way of characterizing this behavior. In many cases, forehead-snapping is a mild form of aggression or teasing, and the terms "brain-snapping" and "forehead-punching" are a good way to visualize this behavior.