[Dabhand16]

A slave trigger is like a table tennis ball - sometimes half black - that plugs into the slave flash unit. It fires the slave flash when it detects the main flash (controlled by the camera) firing. The main flash can be the on-board flash or one directly connected to the camera either on the hot shoe, or by a sync cord.

If using the on-board flash, make sure that the red-eye reduction is switched off, or else this will fire the slave and not the actual main flash discharge.

Some cameras also use infa-red or radio transmissions to control slave flash units that have to have a matching system.