In Bomberman games that feature the ability to lift bombs off of the arena in any way (such as punching or throwing), players hit on the head can be stunned. This affects the player in various ways depending on the game, and this effect can be induced in different ways by different attacks.
Types of Stun Effects
In most 2D Bomberman games, the stun is a short term effect induced by being hit on the head with a bomb. Typically, the character spins around or does a "stunned dance", after which the player regains control. In most games, some of the player's items will also be scattered across the arena.
Another form of stunning is when the player is knocked off of their feet. First seen in Bomberman 64, the character sits idle for a rather long time, but by rotating the joystick or pressing certain buttons, this "stun time" is reduced until the player awakens, invincible for a brief period of time.
Methods of Stunning
Throwing a bomb on another player's head will most certainly guarantee that the other player will become stunned, unless the other player is holding a bomb above his/her head, in which case the thrown bomb will simply bounce off. Some characters, such as Hammer Bomber and Lady Bomber, have special attacks that will stun players and scatter their items. Some rideable creatures also have stun-inducing attacks, most notably the "Music Dancer" from Super Bomberman 4 which fires some notes that will cause hit players to break out into a short dance.
Some enemies and bosses also have stunning abilities, which vary. In Bomberman 64, most enemies will not kill Bomberman upon touching him, they will only stun him.