Flashes can be triggered in several ways: via a synchro-cable connected to the camera, by a built-in photocell triggered by the flash from another unit, or by being interconnected to another flash via a Synchro Interconnection Cable. This cable must be used when "daisy-chaining" or wiring together packs because it is opto-isolated, preventing the dangerous build-up of current in the synchro-circuit.