The Mechakoopa marches until colliding with an enemy, knocking them back before exploding. It does 2% when jumping and 7% explosive damage when it explodes for a total of 9%. It deals weak diagonal knockback. If the Mechakoopa walks to a ledge without hitting anyone, it will turn around, and if it isn't activated after about five seconds, it will explode on its own. Bowser Jr. can only have one Mechakoopa out at a time; the move will fail if he uses it again before the Mechakoopa is gone, leaving him with around half a second of lag. In Ultimate, if a Mechakoopa collides with a shield, it will switch off and become an item the opponent can pick up.
If a player picks up the Mechakoopa, it will reset the timer. This can be used for combos or potentially dropping off-stage for gimping opponents. However, the Mechakoopa counts as an item that can be picked up and thrown back at the user, similar to Diddy Kong's Banana Peel. As a result, these plays are not reserved to Bowser Jr., and they're equally effective against him as they are on the opponent.
When a Mechakoopa jumps onto an opponent, it will latch on for a limited period of time before exploding. This can be exploited by the Bowser Jr. player in a number of ways: "extending" throw knockback, combos into forward aerial and confirming smash attacks among other things. This is a massive part of Bowser Jr.'s playstyle, and learning how to use it is imperative to learning the character.
Special Move customization was added in Super Smash Bros. 4. These are the variations:
Mechakoopas debuted in Super Mario World, appearing in Bowser's Castle and being thrown by Bowser out of his Koopa Clown Car during the final battle. Mario can disable them by stomping them, though they will return to normal after some time. When they are deactivated, Mario can pick them up and throw them; this is the only means of defeating Bowser in that game. They appeared mostly in spin-off games over the years before returning in New Super Mario Bros. Wii.