What Are Slave Units?

A Slave Unit is a device that is sensitive to sudden light changes that acts as a switch.

A Slave Unit can be attached to a flashgun with a cable or via a hotshoe and provides a means of remotely triggering a flash gun that is within range of the "Master" flashgun.
By strategically placing multiple flashguns that are triggered in this manner, shadows caused by the light source can be greatly reduced or eliminated entirely.
Another useful reason for using a Slave Unit is that older style SLR flashguns can be used in conjunction with newer DSLR flashguns without fear of damaging the camera or flashgun circuitry. I regularly use my old Miranda 500 CD and Miranda CTB-24 flashguns in this manner.
Most Slave Units come with a threaded base that is handily compatible with tripod mount threads, or with a sucker that can be stuck to most flat surfaces to enable best positioning for the flashgun.

Advantages:

• Slave Units can be used with almost any flashgun.
• No power required.
• Slave Units can be positioned almost anywhere.
• A relatively cheap commodity.

Disadvantages:

• Slave Units can cause premature firing of the flashgun, especially when red eye reduction mode is employed as this uses pre burst flashes to close the eyes retinas.
• Slave Units can have a limited range, dependant on the "Master" flashgun power.
• Cheaper units tend to be unreliable and have a shorter operation range.

Warning:
Most older SLR camera flashguns are not compatible with modern DSLR cameras and if they are used directly on the DSLR camera hotshoe or cable link it could seriously damage the electronics of the camera.