A Primid careening to the right in Brawl.
Careening (referred to in animations as "BulletMode") is a state entered by defeated enemies in the Subspace Emissary. Once its HP reaches zero, a blast sound can be heard and the defeated enemy begins flying in a straight direction (based on the angle of the attack that hit it), bouncing off parts of the stage and damaging other enemies it runs into. Upon reaching a blast line or after a few seconds the enemy is removed from play.
The damage dealt by a careening enemy is dependent on the difficulty, with higher difficulties making the attack weaker. Generally, larger enemies deal more damage when careening. Careening enemies also always use a special knockback angle encoded as 363, whose properties are mostly unknown.