External triggers can be used to extend camera(s) capture capabilities like bulb mode for cameras which don't support it or to trigger multiple cameras in same time without lag. For this example a cheap usb relay is used from ICstation http://www.icstation.com/icstation-micro-control-channel-relay-module-icse013a-p-4013.html same relay can be bougth from ebay also.
To use an external device first it's required to define it. This can be done in Settings->Devices->Add. The image below shows the settings for 2 relay combinations more in Settings->Devices.
After a device is defined, can be used in multiple places like in Multiple Camera control or Astro module both have a dedicated section for external devices. Another usage possibilities are to attach an external device to a camera using camera property.
If a camera has an attached device, all capture action will be executed by the device not by the normal camera capture same behavior for trigger using using script or remote utility. In this way if to the relay is connected to 2 cameras and if we click on capture both cameras will capture.