A crush is a very uncommon method of being KOed, which involves the player's character being stuck between two solid platforms that move into each other; if the player is unable to move out of the way from the common collision point of the two platforms, then they will be crushed when the two platforms meet. This results in an instant KO regardless of the player's accumulated damage.
Stages that can crush
Thus far, only one versus stage in the series features crushes: Midgar.
In Midgar, summoning Odin via Summon Materia on the stage will cause him to slice the stage in half. After some time, the two halves of the stage will come back together; players caught in between these two halves will be crushed.
In the Subspace Emissary
Outside of the above, Brawl's Subspace Emissary features several levels with crushes in them; owing to the mode's focus on platforming, the player is tasked with correctly timing their movements as to avoid these crushes. The following stages have crushes: