Gargoyle is correct about the "cue marks" being five seconds before the cut.

There were actually two sets of circular cue marks - one set etched in the corners of four consecutive frames of the film with a little pin at a standard measured distance before the cut, and the second set etched about six inches before the cut.

When you saw the end of a reel approaching you struck up the carbon arc on the projector you were changing to and then kept watch for the first set of cue marks. When you saw the first cue marks you quickly started the motor on the projector you were changing to, so it could get up to speed, and then, when you saw the second set of cue marks, nearly on top of the cut, you flipped a knob which mechanically lowered a metal shutter (or dowser) from the light path on the outgoing reel and lifted a shutter up on the incoming reel.