Fun fact: A not entirely equivalent, but comparable starter was used on the Titan II ICBMs and Gemini launchers, specifically the LR87-5 and LR87-7 rocket engines (which used a hypergolic fuel&oxidizer combination)
A simple (read: I don't understand it better yet) explanation is this: A starter cartridge was placed in the turbopumps of the engine, that ignited and spun up the turbopumps (technically, the turbine of the TPA which then turns the pumps), which then fed fuel and oxidizer into a) the gas generator to sustain operation, and b) into the combustion chambers. The cartridges burned for about a second, and you can hear the screeching sound they made in this launch video [1]