By the time the combustion was complete the pressure in the flame cans would be much higher, although the actual expansion ratio is not specified. The hot gas was then released, and flowed through a turbine to extract power, instead of forcing a piston to move (although most of the output in the expected turbojet engine format - as opposed to a turboshaft  - would be extracted as thrust, and the turbine only acted to power the compressor to continue the cycle).
The combustion process was complete when the pressure inside the flame cans reached a high level, and the hot gas was then released to flow through a turbine to generate power. This process differed from directly forcing a piston to move (although most of the output would be used for thrust in a typical turbojet engine format - unlike a turboshaft). The turbine only powered the compressor to keep the cycle going.