When vacuum is absent, the opener will push the 4 butterfly to open a little bit. This is similar to lightly press on the throttle. So when the moment we start the engine, the rpm surge to 2500rpm.
When vacuum is present (engine started), the vacuum will suck down the opener's rod and the 4 butterfly will be at completely closing position. At this point (idling), the engine breath through ISCV. This explain why after start the engine, the rpm will move down to 800~1000rpm.
So its similar to auto choke, right?