The shortcut is on the Wuhu Mountain Loop track (called Maka Wuhu in the US version) and if you know where it is you can cut out almost half the course.
It's not meant to be there, and it's clearly not an easy thing to find - given the game originally came out over a month ago - but Nintendo is refusing to patch the game to get rid of it.
'We are aware that it is possible to navigate a certain part of the track in Wuhu Island in a way that allows a large part of the course to be bypassed,' admitted Nintendo of America in an email to website Sticktwiddlers
'There are no plans to update the game to remove this shortcut as doing so would create an unfair advantage for the users of the original release of the game. Rest assured your comments have been added to our records for Mario Kart 7.'
Although it's rarely done both the Wii and Nintendo 3DS can receive patches if Nintendo deem it necessary, the most famous example being the bug in the recent Legend Of Zelda: Skyward Sword.
If there not willing to do that though we'd just be very wary of playing anyone online who constantly votes to play that level, and then mysteriously always wins…