The soul breaker is a glitch in Super Smash Bros. Melee performable only by Mewtwo. It involves using Confusion to simultaneously grab an opponent and reflect an item that is too strong to handle, resulting in Mewtwo having its reflector broken and the opponent being stuck to Mewtwo until it is KO'd.
The easiest way to perform the glitch involves a Red Shell. Using Confusion on a Red Shell five times will make it strong enough to break the move on the sixth reflect, during which the opponent is grabbed. It is possible to activate the glitch with other projectiles, though the required timing and setups are much more difficult.
While the victim is attached to Mewtwo, they cannot do anything except take damage. There is no way to free the victim without KO'ing Mewtwo.
While the soul breaker glitch has game-breaking potential similar to the freeze glitch (which is banned in tournaments), it is less of a threat in actual matches since its specific conditions (catching an opponent with Confusion while reflecting a projectile back and forth) make it impossible to achieve unless all players wanted to intentionally provoke the glitch, which is far aided by Confusion having little practicality in competitive play and all items including the Red Shell being banned in tournaments.
The glitch is the result of Mewtwo having its reflector broken while grabbing another character; with the animation interrupted, the code to release the victim is never reached, and since Mewtwo was not attacked and damaged the general "release victim when hit" code is not run (since the start of a shield/reflector break is a different state altogether). As Mewtwo is the only character with a move that grabs and reflects simultaneously, no other character can use the glitch.
An odd side-effect of the soul breaker is called the soul stunner. While the soul breaker is activated, using Mewtwo's down throw on another player will freeze them too. Though similar to soul breaker, the soul stunner doesn't cause the victim to follow Mewtwo around, and will instead remain frozen in a certain spot. By charging Shadow Ball on two frozen characters, one can obtain an infinite combo. The soul stunner can be negated by using Confusion on the frozen character.
This is likely due to conflict over which character Mewtwo is currently grabbing - as normally one can only grab one opponent at a time, the game is unlikely to try and apply all the necessary code to multiple grabbed characters. It is therefore unclear why this doesn't occur with all four throws, though down throw does have the distinction of being Mewtwo's only multi-hit throw.