The period after a fighter has received a hit when they’re flinching, unable to perform any actions. No inputs are possible for a certain period of time except giving control stick input for hitstun shuffling (while being hit) and directional influence (during a launch). Any other hits that connect during this period result in a guaranteed combo, racking up lots of damage.
When launched into the air, helpless and tumbling, a fighter is stuck in hitstun for a certain variable period of time, dependent on the knockback of the launching attack and the damage already piled up on the launchee. Before that point, all that can be done is control stick input to bend the launching direction, cutting the momentum with directional influence. At a certain point, though, the “locked” hitstun period ends, and the tumbling fighter can steer their helpless body, regaining full control by dodging or attacking, or teching upon contact with the ground.