From the Super Mario Wiki
The Sliding Haymaker, also called the Sliding Punch, is one of Bowser's moves in Mario & Luigi: Bowser's Inside Story and its 3DS remake, Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. Bowser learns this move after Mario and Luigi defeat Durmite in Bowser's Nerve Cluster. They head further in the area when a lot of Biffiduses drill down the ice the brothers are currently on, dropping them onto a small area. In this place, they find Toadbert, who is studying a nubbin, which Luigi hits, giving Bowser a tingling feeling in his arm. Toadbert tells them what he believed happened. With this move, Bowser is able to continue his journey.
To perform the move, the player must punch normally using the button, and hold down the in the direction they want Bowser to move. This move can be used to cross ledges that Bowser would normally be unable to cross on his own (called "Slide Fit" by a sign in Bumpsy Plains or "Treadmill Fit" by a sign in Bowser Path) and also to break bigger rocks.
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