The Underdog Boost is an optional rule in Super Smash Bros. Ultimate that helps to provide balance in matches between human players of different skill levels. When enabled, players will be given multipliers to increase the damage and knockback they deal, and decrease the damage they take, based on how far behind they are from the player placing first in the match. It can be enabled or disabled in the "Advanced" section in Rules, and is turned off by default.
The multipliers given are unique to each of the three stats—damage dealt, damage taken, and knockback dealt—and are also affected by the amount of players in the match. Generally, the lower the player's rank relative to the first-placing player, and the fewer players there are on the match, the higher the multiplier becomes. However, the multipliers for each stat are also capped to a specific value, preventing them from growing to ludicrous amounts.
The multiplier for each stat is determined by the formula
For example, in a two-player match, if one player has two stocks less than the other, the calculation for the Underdog Boost stat multiplier is equal to
The damage dealt multiplier is ignored entirely by the knockback formula, much like the 1v1 multiplier, so only the knockback multiplier will affect knockback.
Possible stat multipliers
The following tables showcase the stat multipliers for each combination of game mode, rank, and total players.
In competitive play
The Underdog Boost is universally banned in all tournaments, as it gives losing players an unfair advantage by progressively making them stronger the further behind they are. This compounded with the universal Rage mechanic makes even a one stock difference an uphill battle for the winning player. Fighters like Lucario, Terry, Sephiroth, and Kazuya particularly benefit from this boost, as they already have special comeback mechanics built into their movesets that only get more powerful with the boost on.