The article doesn't actually say what is does, just that it's function is to ensure the loading of film occurs at a constant speed which, it is implied, keeps the audio in sync. It doesn't say how it does it, how it's connected, or whether it has a true control function or is just a monitoring device which is used by the projectionist to make alterations (using another part of the system) should the operation de-sync. The only operator they talked to claims to neve have interacted with the device.