In order to pull this short cut off you must have a light weight kart with atleast one mushroom in your arsenal. When you are coming around the corner just before you leave Yoshi's nose you will see a gap in the fence. This gap allows you to drive of the road, over a cliff. If you activiate your mushroom just after you leave the cliff you will be propelled across the air into a tunnel, which can cut almost a whole second or more off your time.