On-screen appearance

The on-screen appearance, also known as an intro or entrance, is a short unique animation for each character entering the battlefield during the three second countdown before battle. It's a minor feature in the main Super Smash Bros. games with the exception of Super Smash Bros. Melee. It is present in Super Smash Flash 2 as well. It has no effect whatsoever on game play other than visually dramatic and comedic purposes, with the exception of Wario, who's Wario Waft begins charging the moment he enters the stage. Each entrance relates to that character's universe, such as Mario entering in through a Warp Pipe.

In Super Smash Flash
Like Melee, characters didn't have unique entrances. In Melee characters start out as trophies that come to life at the start of the match. However, in Super Smash Flash, characters hover in the air before dropping onto the stage.

In Super Smash Flash 2
Like all of the games with the exception of Melee, characters have unique character entrances. During the time when on-screen appearances are occurring, the game cannot be paused.

Trivia

 * Wario is currently the only character who has a different on-screen appearance from the one he has in the main Super Smash Bros. games. In the main Smash Bros. games, he drove onto the stage using the Wario Bike, while in SSF2 he drives in the Wario Car.