Mario Kart Wii sets out its stall in the last corner of the first track in the Mushroom Cup, Luigi Circuit. There's a chicane, followed by a huge, wide, banked curve, which leads onto the finishing straight. Inside the chicane, off the track, is a ramp. The top of the banked curve is lined with a series of ten or so on-track zip pads.
So, if you have a mushroom item in store, the quickest route around the corner is this. You use the mushroom for a speed boost, short-cutting across the chicane. You hit the ramp and jerk back on the wheel, executing a stunt. This gives you a speed boost when you hit the track again. Then you hit the first zip pad, and get a speed boost. You hop and slide, counter-steering with the wheel, sparks flying, first blue, then yellow. You hold the slide as the zip pads give you speed boost after speed boost after speed boost. You hit the straight and come out of the slide - which gives you a speed boost.
If you're really lucky, you'll have been doing all this in line behind another racer, slipstreaming granting a speed boost. If you're on a motorbike, you can then pull a wheelie down the straight. This gives you... you get the idea.