Shield break
Each character has a shield with a particular size and strength. With each hit inflicted within a period of time, the shield will diminish until it is broken. This results in the character being thrown into the air, and being stunned for a few moments after landing. This is known as a shield break.
The purpose of a shield breaking is, along with grabbing, to prevent a player from staying in his or her shield indefinitely. Instead, the shield will get smaller and smaller, exposing more of the character to attack until the entire shield is depleted. Shield breaking moves are either ones that can be comboed to prevent a character from rolling out or from dropping the shield without being hit or ones that deal enough shield damage to break the shield in one hit. Certain moves, like Marth's fully charged Shield Breaker, will break any shield in one hit. The items Fan and Mr. Saturn are also very effective shield breakers as well.
If a character's shield breaks while at the very edge of a stage, the possibility of the character being stunned until falling into a KO can occur. The knockback of some moves when breaking the shield transfers to the opponent, thus pushing the character with the broken shield off the edge.
Players who use Jigglypuff should be especially careful about is its shield breaking. Like other characters, if Jigglypuff's shield breaks, Jigglypuff will fly upward and then be stunned for a short period of time. Unlike other characters, however, Jigglypuff flies several hundred times farther upward than normal. This is well enough to kill Jigglypuff, unless there is a ceiling or some other obstacle preventing it from flying straight up.