The 4030 Slave Unit is a terrific little electronic device that can fire a flash unit when it "sees" another flash unit go off. Any number of slaves can be used to fire an unlimited number of flash units. Just plug the 4030 into any sync outlet on a power pack or any other lighting panel. Tip: The trigger is polarized. If you plug it in, and another flash fails to fire it, unplug it and flip it around 180 degrees. That should do the job. **Caution: Never attempt to plug this device into any electrical wall outlet or extension cord. The A/C power will destroy the Slave Trigger possibly starting a fire and electrical shock hazard.