From the Super Mario Wiki
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A Shell Dash is a move which Mario and Luigi can perform in New Super Mario Bros. They can only do it if they are in their shell form after collecting a Blue Shell. It can be performed by running for about two seconds. After doing so, Mario or Luigi will duck into their shell and dash as fast as a green shell or red shell when it moves. In this form, Mario or Luigi can jump, break Brick Blocks and defeat enemies. Unlike kicking a Koopa Troopa's shell and hitting multiple enemies to earn increasing points and extra lives, Mario or Luigi only earn 200 points for every enemy defeated when using a Shell Dash. In Mario vs. Luigi, this move can be used against the opposite bro.
There is a similar move in Super Mario 3D World. If the player's character holds a shell, once he or she crouches, they duck into the shell and dash. The player can exit the shell any time; if they do so, they'll do a Spin Jump. Unlike in New Super Mario Bros., one can earn increasing points and extra lives with this move. If the player hits any team members, the member will power down or lose a life, depending on their form. After a while, the player's character automatically escapes the shell, becoming dazed and stunned for a few seconds.