A cross-up is the act of timing an attack (typically a dash attack or aerial) such that the user moves past the opponent and ends up behind them once the hitboxes are gone. Dash attacks are the more common kind of cross-up as the user is moving quickly already and many give the user additional propulsion. Short hop Back aerials are a common cross-up as well, with the user starting in front, then short hoping over the opponent, followed by a back air as they go over the opponent. Cross-ups could be considered a kind of approach, but they can also be used to perform mindgames. The idea of the technique is that the attacker cannot be shield grabbed when behind the opponent, which makes such attacks safer to use by removing a common option from the defender. However, some characters have moves that strike faster against targets behind them, such as Kirby's up tilt or Mario's up smash, making the technique risky in such cases. Attacks that can be used to cross-up include Fox Illusion and the dash attacks of Snake and Yoshi in Brawl and Smash 4 (respectively) - with proper spacing, these attacks can put the user behind the opponent and end with the user in a less vulnerable position for punishment. This technique can be done repeatedly to rack up damage if the opponent doesn't react properly, although it can still leave the user in an extremely vulnerable position if abused.
In Super Smash Bros. Ultimate, the majority of attacks that could perform cross-ups can no longer do so, as players cannot run through opponents. As a result, many momentum-based attacks such as Fox Illusion and Heel Slide will stop against shield, making this strategy useless. However, there are still some attacks that can perform cross-ups if used closed enough to a shielding opponent; these include Simon and Richter's dash attacks. With the use of a roll cancel boost grab, some characters can perform a pivot grab that causes them to end up behind an opponent; however, this is dependent on the character.