Withdraw
Withdraw is Squirtle's Side Special Move. Squirtle goes inside its shell and speeds across the stage, powered by a jet of water shooting out the back of its shell. During this attack, Squirtle behaves like a Green Shell item for a limited amount of time. If another player jumps on Squirtle, the attack ends and Squirtle becomes stuck on its back for a few seconds. Squirtle cannot take damage during withdraw. Unlike Yoshi using Egg Roll, Squirtle can't change direction on its own while using Withdraw, but just like the shell item, it will turn around if it hits a wall, and can change speed and direction when attacked. Spinning up and down hills can alter the speed in which the withdrawn Squirtle will move. The attack can be cancelled if the control stick is moved at the opposite side that Squirtle is attacking. The invicibility provided by the move is sufficient to block many Final Smashes.
Origins
Withdraw is a move in the Pokémon games that makes the Pokémon withdraw into their shells, raising their defense. This being so, only Pokémon with shells can learn it.