It's an optional primer pump. Very useful when flying in the winter time in colder climates. Flew the 44 a lot for pipeline inspections and below 0°C it was a real pain to get the Raven 1 to fire up in the winter in the morning. With the primer pump you unlock it, pump it a couple of times until it draws fuel (you can feel it in the movement of the stroke) and after that pump 2-3 times. Much more efficient than twisting the throttle 40-50 times to get it to fire maybe once and die again straight away. When it's cold you don't have many attempts before the battery dies. Also very helpful to pull all the CBs of unnecessary electrical consumers for starting and to have a 2nd guy turn the tailrotor/ drivetrain whil you're cranking.