I don't think there's any danger there - the most that might happen is that you might get wet!

My son did this as part of a physics class project in high school. As I recall you pumped the air in while the bottle was locked on the stand, then you "pulled the trigger" via a string from a "safe" distance away. The lock/trigger was a u-shaped piece of plastic that locked onto the ring that is just under the cap of a two-liter bottle.

(Anybody know what that ring is for? At the bottling plant, the bottles, before filling, are conveyed along by hanging them by that ring in a sort of tube with a slot in the bottom. The tube is arranged with a bunch of angled jets on top that blow compressed air, and the empty bottles are just blown along the tube.)