From WiKirby, your independent source of Kirby knowledge.
Crash Rush is a special Friend Ability which allows Crash Kirby to send his Friends flying at high speed after using a Crash explosion, causing them to act as projectiles to deal additional damage after the initial blast. The effectiveness of this ability depends on how many friends Kirby has with him, with a full party being optimal.
Once the friends finish being flung around, they splat onto the screen before reappearing on stage, similarly to when Kirby hits the screen in Kirby: Triple Deluxe.
Kirby's friends are knocked into the fourth wall during a battle with Whispy Woods.