How does cruise work?

[nick lawrence]

Specifically the vacuum portion. 
 I guess the the electronic portion uses a set number or voltage then compares to signal from speed sensor.
My question is about the vac portion. Does the vac motor run all the time and use a bleed of sytem to position the throttle?
Or does the vac motor run when it needs to pull open the throttle? Seems it would still need a bleed off to change or quickly close the throttle?
Anybody know for sure?