Three PocketWizard Plus II 4-Channel Auto Sensing Smart Transceiver. Condition is Used. Shipped with USPS Priority Mail. Transmitter or Receiver in One Unit Works on U.S. Frequency: 344MHz Fast Plug-and-Play Set-up 4 Selectable Digitally Coded Channels The PocketWizard Plus II Transceiver/Relay is a triggering system that offers Auto-Sensing Transceiver Technology. This technology analyzes the status of the Plus II's miniphone jacks, camera hot shoe or the TEST button to determine which mode (Transmit or Receive) to set itself. By default, the Plus II is always in Receiver mode unless it senses a trigger pulse from a camera's hot shoe. It then quickly switches to Transmitter mode. The Auto-Sensing feature can also be disabled, if necessary. This unit can also be used as a relay. You can remotely trigger your camera, and the camera-mounted transceiver automatically relays the triggering signal to a remote flash. With the Plus II's fast switching speed, there's no loss of camera shutter speed performance or triggering delays.