The health bar is an optional mechanic in Super Smash Bros. Battle Royale.
Unchanged from past Smash games, you K.O. your opponent by knocking them off of the stage, be it vertically or horizontally. However, unlike past Smash games, each character also has a health bar. When the health bar depletes completely, the character moves slower, deal less damage, be K.O.'d easier, and, overall, become worse. Each characters have different health bars. You can restore these gauges by either switching to your partner, should the mechanic be in use, or by eating food. Without items or partners, the health bar will restore 1% of the overall health bar every 3 seconds. 70% of the health bar is replenished when K.O.'d.
In tournaments, this feature will most likely be overlooked. While interesting, it's too heavy of a nerf for most characters.