It spins, it creates a pressure difference, and air moves. The fan does not decide the airflow by itself — speed and system resistance fight over the result. Drag both and watch the fight.
The damper stands in for everything downstream and upstream of the fan: duct, fittings, coils, filters, diffusers. More restriction = a “tighter” system.
With mapping on, every state you settle in drops a dot. Sweep the damper at a fixed speed and the dots trace a single line — that line is the fan curve. This is literally how manufacturers make one: an AMCA 210 lab rig holds the fan at constant speed, throttles the outlet step by step, and measures each point.