The vacuum/ air velocity is at play as the blades close, if you do not believe it drive 70 down the highway, stick your hand out the window flat, then turn it 90 degrees. Let me know what happens, same principle. The throttle shaft bore is offset like .030 from the throttle plate bore so there is always a slight pull to close it, and keep it from slamming wide open. A 6bbl with vac pods will never open or close instantly. If I start my 6bbl there is enough play in the linkage that you can slightly open the 2 end carbs, not move the center carb and spring, and magically it returns to idle, all while running, I could care less what It does when the engine is off. It has to be the vacuum acting on the .030 offset of the throttle blades that creates more force to close than open. I could be all wrong here.