Sandbagging, sometimes formally referred to as "intentional underperformance", is the act of playing poorly on purpose. Originating from Billiards slang, referring to the concealment of one's ability to play, sandbagging in the Super Smash Bros. series generally consists of players using characters that are not their main characters, selecting stages that normally counter their character, or playing below their proven capabilities, amongst other actions. Sandbagging can occur at any level of play, and even at top-level tournaments players have sandbagged on occasion, with some smashers, such as Isai and Mew2King, being well-known for their frequent sandbagging.
Sandbagging is sometimes done in friendlies in order to prevent spectators or opponents from learning their playstyle with their mains. If the skill difference between two players is particularly significant, higher-skilled players can also potentially sandbag. The reasoning for sandbagging can vary, though in both cases, it can be used by players to relax in lower-stress matches, as well as potentially allow for an opportunity to work on other characters; in the case of differences in skill, the sandbagging player can also give their opponent a "fairer" match.
Sandbagging, however, can be seen as a sign of disrespect when done in serious settings, as it is seen as the player not treating their opponent as an equal nor as a threat, as well as depriving their opponent of a pure competitive match. Due to compromising a tournament's competitive integrity and disrupting the natural progress of the tournament bracket should a sandbagging player lose against a lower level opponent, it is not uncommon for tournaments to have a rule forbidding intentionally underperforming at the threat of being disqualified from the tournament altogether and possibly banned from future tournaments. This is a controversial ruling though, as it is not always clear if a player is sandbagging, thus making it difficult to enforce. Additionally, there is the argument that it is the right of the player to perform however and use whatever character they want. There is also the argument that a player may simply enjoy playing as the character even if they do not normally play them seriously.
Notable incidents of sandbagging