The only thing attached on the outside is a small cap like head. It is actuated by the throttle going to full position. During Start, the throttle is held and the start button is pushed. This allows the engine to spin up enough to fire and when the start button is released I guess the person driving the weed eater lets off the throttle ???????????

I do know it it operated by the throttle. If you look at the first picture you can see that when the throttle rotates it contacts the CR.