Flash slaves are used when you need to supplement one flash unit with one or several more. This slave trigger simply triggers those other units. It does this by `seeing` the first flash (using a phototransistor) and triggering the other flashes a few microseconds later. The sensitivity of the circuit is adjustable to compensate for ambient light or dimmer than usual master flashes. Q1 should be aimed at the master flash. R1 should be adjusted for maximum sensitivity but no false triggering. Keep your fingers away from SCR1 when the circuit is operating. It is at 50V-300V and will give you a nasty shock.