The Mario Tornado is the name given to Mario's Down B. His down B spins him around in a circle rapidly, and can be used for recovery as well. To do this, press Down B in the air, and rapidly keep pressing B. You should gain a good amount of height if you have medium-fast fingers. To test if you are getting good height, use it on the ground. If you can double jump after the tornado, you are doing it well. It is a multi-hit attack, and the first who hits don't stun (phantom hits). Here is some frame data:
Mario Tornado
Total: 79 Hit: 8-9, 12-13, 15-16, 18-19, 21-22, 24-25, 27-28, 38-39
Credit to SuperDoodleMan for that frame data, and all his amazing work.