From the Super Mario Wiki
Thunder Tickle, also known as the Shuffle, is a special move in Mario & Luigi: Superstar Saga. It is taught to Mario and Luigi by the Jellyfish Sisters at their Relaxation Room in Gwarhar Lagoon. Gigi and Merri decide to teach Mario and Luigi how to use the Fire Tickle and Thunder Tickle after the two brothers inadvertently reveal that they can use Firebrand and Thunderhand. When Luigi uses his Thunderhand ability while behind Mario, he shocks Mario, briefly exposing his skeleton and causing the two of them to walk in parallel. While Mario is being shocked, the two are forced to move at the same time and not change direction until it wears off. They move slower than usual, but the move allows them to move through certain gaps at the right moment and interact with objects that require them to be facing away from them, such as Boo Statues. It lasts for about four seconds before wearing off.