Lets you know, positively and at a glance, that all of your remote flash has fired. Simply stick a cordless TeleFlash onto each flash in your setup, and it will detect a good flash firing (through the electro-magnetic surge) and blinks a bright red LED for 10-15 seconds, then automatically resets for the next shot. One 3V lithium battery (CR123A, included) provides power for thousands of activations.