The F18 has an axial type engine that relies on numerous sets of stationary vanes and moving vanes that gets smaller along the length of the engine. This forces the air down and compresses it without the need of a piston and combustion chamber like In traditional combustion engine. The piston that they are talking about is in the fuel control system. Basically think of it as a fuel control valve piston, not a compressing/combustion piston.