It takes two paragraphs to explain it in the article because IMAX is really complicated. The important thing is that it's mainly a monitoring device and projectionists usually just have to leave it alone and let it do its thing.

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.