It's very complex.
But in it's most basic form, there are 4 pistons that are as long as the tower is tall(smaller tubes inside the tower) air is pumped into those (from the Shot Tank which is the large tube in the middle of the 4 smaller tubes in the tower). That's really it. I remember for a while there was a really good detailed explanation on the S&S website. It doesnt seem to be there anymore though.
Favorite Wood Coasters: The Voyage, Ravine Flyer II, Thunderhead, Balder
Favorite Steel: Voltron Nevera, Steel Vengeance, Expedition GeForce, Olympia Looping
Parks visited: 232, Coasters Ridden: Steel: 894, Wood: 179, Total: 1073