Tumbling usually occurs right after the hitstun of a launching attack. Tumbling fighters are noted by their rotating animation. Hence, tumbling. Tumbling is notable in that while it looks like you’re in a combo-ready hitstun state, it’s more like a kind of limited defensive drifting posture.
While tumbling, you can adjust your fighter’s aerial trajectory, perform any aerial action to cancel the tumble, tech (if close to a surface), and grab nearby ledges. If you hit the floor while tumbling, you transition into a knockdown state, allowing the enemy an opportunity to hit you with an off-the-ground attack.