From the Super Mario Wiki
Mario doing a Power Squat Jump
The Power Squat Jump is a jump that Mario, Luigi, Princess Toadstool and Toad can perform in Super Mario Bros. 2 and its remakes. It is done by pressing down on the Control Pad, and then charging the jump. Characters do not necessarily have to remain ducking; they can keep the charge until they actually jump. Unlike the normal jump move, this jump makes the characters jump higher and reach places they normally can't. In the Super Mario Bros. 2 remake Super Mario Advance, the characters gained the ability to perform a back flip upon using the Power Squat Jump.
Although this move did not appear in early versions of Mario Bros., later, in the Mario Bros. remakes found in the Super Mario Advance series (and Mario & Luigi: Superstar Saga), Mario and Luigi could also perform this move.
In Super Mario 3D Land, Mario and Luigi were able to perform a chargeable jump upon crouching that works very similarly to the Power Squat Jump. This feature returned in Super Mario 3D World.
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