[Christ]

Yeah it lets a little air into the diaphragm... but it's not how you're thinking.

If it's under vacuum, and you let some air into it, it's just under less vacuum.

In order for airflow to affect the situation, it would have to be under no vacuum.

Remember, there is a spring that applies tension to the throttle cable.

Lets say that spring is 3lbs of force (twisting). There must be enough vacuum maintained to allow the diaphragm to defeat that 3lbs of force, before any throttle actuation occurs.

How does this affect the outcome? The diaphragm can still be under vacuum, but not have enough pull via that vacuum to actuate the throttle. To ensure this, there is another spring (a "check spring") in the diaphragm, which further increases the energy necessary to actuate the throttle.

Now, what if your diaphragm has only enough kinetic energy (due to vacuum) to pull 2lbs? You're going nowhere. Up the vacuum a little more, so that it can pull 4 lbs, and now you're getting somewhere... you've opened the throttle (but just barely.. remember, springs aren't linear).

Now lets say the "brain" of the CC realizes that you're in an overspeed condition:
Release 2 lbs worth of your vacuum, and you're back to the 2lb pull that isn't doing anything.

The "check spring" I mentioned earlier is there to ensure that the diaphragm incurs more work than your foot does, so that response and pedal feel isn't changed while the CC is active. It makes it so that the diaphragm (which maintains vacuum while it's active) has to pull the throttle harder than your foot would. The diaphragm maintains enough kinetic energy via vacuum to slightly depress the "check spring", but not apply pressure on the cam spring for the throttle. This means that your foot feels exactly the same pressure regardless of the active state of the CC system.