The control relay gets energized when the infra red light emitted by the 2emitters at the top and bottom are received by the respective receivers. The relay remains energized even after the bottom receiver stops receiving the light. However, when the infra red light emitted by the top emitter gets cut and is not received by its receiver the time starts and after set time the relay reverts to original position (NC).
The relay once again energizes when both receivers start receiving the infra red light.
