Exactly what you observed. It is an "accelerator" pump that squirts a quantity of fuel into the throat of the carburetor when you advance the throttle. If you look at the port side of the carburetor, (opposite the throttle linkage) it has a small chrome shaft sticking out of it, linked to the throttle linkage by the same shaft that runs the butterfly valve inside the carburetor. It will move down when the throttle is advanced and up when retarded (pump, refill). Some pilots have no primer and use this to prime the engine when starting.