When an engine starts up, it needs plenty of air to keep it going. The air mixes with the fuel and that is what fires the pistons and allows the car to function. Sometimes the air coming in tends to pull the choke closed. In order to keep it open so the engine can get sufficient air, the choke pull-off holds it open. The choke pull-off is a vacuum operated diaphragm. In some models, there are two choke pull-offs to better the air flow. Older models do not have choke pull-offs but have a vacuum operated piston instead. For both, the idea is to hold the choke open slightly and gradually let it open more until the engine is warmed up and the fuel/air mixture is more normalized. If the choke pull-off opens too far or not enough, the engine will probably stall. The choke pull-off can be adjusted by bending the U shaped pull-off linkage.