This shortcut is pretty tricky to achieve, because it is easy to overshoot the path. At the beginning of the course, right before the steep drop, veer to the left and jump. With a little luck, you will land on the course below. Don't get discouraged, this shortcut takes a lot of practice! It is also possible to land on the top of the spiral using this method. You just have to jump earlier.
Mario Kart 64 Shortcut Video Walkthrough Series for Nintendo 64
Mario Kart 64 - Rainbow Road Shortcut Video
