Pressing the special move button makes Ridley rear back and charge plasma in his mouth. He releases a stream of fireballs that bounce on the ground after either holding the button long enough or releasing it. The longer the button was held, the more fireballs he spits out. The move has a long startup, but it charges fully sooner than other charged projectiles, and deals a large amount of damage if all fireballs connect.
While charging, Ridley has a special hurtbox on his mouth. Should he be hit by an attack with hitstun, the plasma will explode in his mouth, dealing extra damage to himself and leaving him momentarily stunned. Ridley will take no knockback, however.
Ridley has had the ability to spew fire in nearly every one of his appearances, though the move in Ultimate specifically hearkens back to its functionality from the first Metroid game, where he spits several fireballs in succession and they bounce along the floor, forcing Samus to carefully dodge them or freeze them to avoid damage.
In the Metroid Prime series, Meta Ridley gains a weak point in his mouth. Whenever he opens it, either to roar or to charge and fire a plasma attack, and the player shoots at it enough, he either becomes dazed and open for attack (in Metroid Prime and as Omega Ridley in Metroid Prime 3: Corruption) or takes damage (as Meta Ridley in Metroid Prime 3). Both of these effects are reflected in Smash.