The 1v1 multiplier, officially known as 1-on-1 damage, is a mechanic in Super Smash Bros. Ultimate. First revealed by Masahiro Sakurai at E3 2018, its purpose is to speed up matches between two players. When a match is started with only two players, and items are off, characters take 1.2× damage from all sources.
Unlike most other damage multipliers, the 1v1 multiplier modifies damage taken, rather than damage given, and is applied after all other calculations. For example, a move with a damage output of 10% first inflicts said amount of damage, and then once all byproducts are determined (such as the amount of knockback), an extra 2% is added by the multiplier, for a total of 12% damage. As a result, it has some unintuitive interactions:
If a match is started with more than two players and items turned off, the 1v1 multiplier does not take place even after there are only two remaining players. However, in training mode, it actively toggles on and off depending on how many CPU opponents are selected to appear (taking effect during any period there is only one CPU), and spawning items does not affect its presence. This also makes training the only mode where damage from non-character items can be altered by the 1v1 multiplier.
The main purposes of this mechanic are to encourage an offense-based playstyle, increase the speed of individual games, and overall amplify the tension of a 1v1 fight. A game where all attacks do increased damage will naturally end quicker and more abruptly. Long games full of camping are slow and often viewed as boring, especially to passive spectators. Games where even weak attacks do high damage and could end at any moment are more exciting and engaging to an audience. This mechanic is meant more for those that are not playing than those that are.