Interlock allows you to play one print of a film on multiple screens. In the case of my theatre, we ran the film from the platter in theatre 14 down through the projector, back up the platter tower, but instead of running it to another platter on the same tower (standard practice), the film ran through another set of rollers across the ceiling to theatre 8, where it ran down the tower, into the projector and onto an empty platter on theatre 8's platter tower. The procedure was reversed for the late show.
Here's some pics:

The automation panel. The blue button is used during interlock.

The cue detector.

The remote status box. This is how I know if there are problems with any of the projectors (other than focus, sound, etc).

Have I mentioned how much fun this job is?