Startup sequence: Fuel and Oxidizer at tank pressure are fed directly into the combustion chamber and ignited. This results in low pressure combustion in the chamber. Hot gas expands and flows out of the nozzle, but upstream of the nozzle throat a tap-off opening allows some of the hot gas to flow to the manifold which feeds the gas turbines for fuel and oxidizer and starts them spinning up.
When the turbines spin up, they begin to turn the fuel and oxidizer turbopumps, which feed Fuel and oxidizer at higher pressure into the combustion chamber. Ignition already took place, so the higher pressure fuel and oxidizer continue combustion and increase the pressure in the chamber. When propellant begins flowing from the turbopumps into the chamber the direct feeds from the propellant tanks close.
The higher chamber pressure in turn flows more hot gas at higher pressure to the manifold that feeds the turbines, increasing their speed and power, which in turn increases the speed and pressure of the turbopumps. This creates a positive feedback loop as increased pressure from the turbopumps increases chamber pressure. A control valve in the tap-off plumbing restricts the maximum flow of chamber gas to the turbines, allowing them to spin up to their full speed but not beyond.