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. The psl-15 is equipped with a ¼” female screw thread enabling you to mount the hotshoe onto a universal light stand or tripod. the falcon eyes 4m4-ur umbrella holder can be combined with the psl-15 to utilize the use of an umbrella to create a separate studio flash unit. alternatively, the falcon eyes cld-15 tilting bracket can be used in combination with the psl-15 to enable you to position your speedlite in almost any direction. both the 4m4f-ur and cld-15 can be found in the optional accessories tab below.