This gadget is a hot shoe flash slave trigger with a 1/4" monoplug that's compatible with digital or analog systems.  It has a control that allows it to ignore a preflash and sync to the secondary (primary) flash.  Perfect for triggering monoplug equipped studio flash from a modern camera with a pop-up flash.  Many cameras that have a preflash for redeye reduction were impossible to sync externally until the advent of this feature.  Sensitivity up to 100 feet away.