The inert effect is used by a small amount of attacks in the Super Smash Bros. series. Inert hitboxes are used for hit detection where a normal hitbox would be unsuitable, dealing no damage or knockback on their own. For example, the starting portion of Captain Falcon's Raptor Boost uses inert hitboxes to detect a target; contact with an enemy does no damage itself but initiates the damage-dealing uppercut portion of the attack.
The inert effect was introduced in Melee, and it has appeared (though remained rare) in every game since.
List of attacks with inert hitboxes