Fivercam Wiki information:

There are three ways to trigger off-camera flash units wirelessly: via infrared, radio signals or a hybrid method that involves both infrared and radio. The infrared system works like the TV remote; there need to be a direct line of sight for the signal to reach the destination. The radio signal has no line-of-sight limitations, but comes at a rather high cost. The hybrid system simply takes the infrared signal from the commander, converts it over to radio and then converts it back to infrared on slave units.