"When you are tuning it, you open and close the throttle and adjust to the point that it closed. When you take your foot off the gas quickly it snaps shut, closing a little further."
That is what Jim said.
Here is a minor edit:
"When you are tuning it, you open and close the throttle and adjust to the point that is needed for the correct idle-speed. When you take your foot off the gas, throttle-flap returns/closes to the idle-speed setting."
I adjust the idle-speed to the idle RPM and then adjust the mixture to maximum vacuum at idle.
Then, re-adjust idle-speed and mixture.
Before/after adjusting the carburetor, adjust timing to maximum vacuum.
That procedure/technique might not be perfect, but it is close and it is simple and it is "fool proof".
