The Falcon Eyes PSL-15 Sensor + Hotshoe is essential if you wish to integrate your speedlite in with your studio flash unit(s) The front of the PSL-15 is equipped with a sensor, which measures the light from your studio flash units and triggers them. By using the TR-1 infrared transmitter, you can trigger your studio flash unit externally. Further, the PSL-15 is also equipped with an X contact.