There actually is a piston in the Stoner system. His patent even calls it a piston.
The bolt tail is the piston and the bolt carrier is the chamber.
As a part of the gas is siphoned off from the barrel and sent down the gas tube, through the carrier key it is chanelled into the bolt carrier.
Once it gets into the Carrier it is now in a sealed environment so it begins to expand and pushes against the bolt tail that is sealed by gas rings, as it expands it pushes the carrier away from the bolt and the cam pin begins unlocking the bolt, one fully unlocked the excess gas is vented via the exaust holes in the carrier.
Read this, its a good cut away to explain how it works.
How the piston in the AR-15 works.
Good video overall and should help alot of new people in dissassembly of their new rifle