Ride Cancelling is a technique that allows the player to prevent a signal from being sent by dismounting a Koopa shell.
When Mario dismounts a Koopa shell, a dismount signal is sent out. This signal overrides all other signals being sent on this frame, including loading and unloading, preventing them from being sent. Therefore, by dismounting the Koopa shell, the player can prevent a signal from being sent.
Its main application is as an infinite coin glitch, in combination with shell cloning. Mario can clone a shell to gain access to infinite uses of the Koopa shell, and can then collect a coin on the same frame as dismounting the shell. The coin does not unload, allowing Mario to return to the shell clone and repeat the glitch.