The photo is self explanatory, but here are some instruction in case you don't read my writing good:
1. Air enters the center tube
2. Air pushes the large piston back, compressing both springs
3. Maximum pressure in, the little spring doesn't maintain the little piston in position, and pushes it backwards very quick
4. main spring returns the whloe system back in position one, and cycle goes on as long as air comes. what do you think????

Edit: The main spring could be replaced by a simple pneumatic ram.
Edit 2 : The air injection tube moves in and out the main cylinder.