Fiber cut simulation typically assumes that both fibers (both Tx and Rx) cut, but in real life, deployments, sometimes the fiber is cut in a way where only one of the strands in the cable is damaged. When this happens, losing one fiber out may not trigger the fail over mechanism because most fail overs rely on optical power monitoring.
ColdFusion simulates this event by mapping two ports in a unidirectional fashion and can be programmed to transmit in one direction, but not in the other i.e. Device B receives signal from Device A, but Device A does not receive signal from Device B eliminating the optical power from a single strand. This can test to make sure the fail over mechanism is triggered in these situations.