Underdog Boost

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.

Mechanics
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, with the variables corresponding to the following:
 * s is a value specific to each stat that determines their base increase: 0.15 for damage dealt, -0.08 for damage taken, and 0.1 for knockback dealt.
 * p is equal to 4.5 divided by the total of players in the match, with a maximum value of 1.2 and a minimum of 0.75. As a result, this variable reaches its maximum with at most three players, and its minimum with at least six players.
 * r is equal to the stock difference between the player affected by the boost and the first-placing player. Additionally, if there are at least five players in the match, the stock difference needs to be 2 minimum, or r is set to 0, resulting in no stat increase.
 * In time battles, this variable works differently. The score difference between both players needs to be 2 minimum if there are less than five players, and 3 minimum otherwise, and the rank value is also halved, so for example, r is equal to 1.5 for a difference of 3 points. This is because KOs between two players in time battles create a difference of two points each, as the attacker gains one point while the victim loses one.

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, resulting in 1.36× damage dealt, 0.808× damage taken, and 1.24× knockback dealt. In a four-player match, this changes to, resulting in multipliers of 1.3375×, 0.82×, and 1.225×, respectively. The resulting multipliers are capped to 1.5× for damage dealt, 0.6× for damage taken, and 1.3× for knockback dealt.

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, , , and 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.

A major incident occurred where Underdog Boost was somehow turned on during, and several important matches were played with the boost on. As too many matches were already played before anyone noticed to course correct, staff were forced to let the rulings stand and continue the tournament with the boost off. It was also turned on for one setup at.