This is a relay power switch for USB devices. It forms a sort of "pass through", with power boost. That is, the device on the receiving end of this device can pull as much current as an external power supply can deliver - not whatever the host can deliver. The switch works really simply. The host provides power to energise the coil of a 5V relay. The relay switches the power from the power supply to the "downstream" side device. The downstream device and host can negotiate whatever they like, can send whatever data they like, but the actual amps come from an external power supply and not the host. The external power supply is never connected to the host, and is disconnected from the peripheral when the host is switched off or disconnected. Note: you can do the same thing with a powered hub. Indeed, such hubs conform to USB standards, where as I doubt this little device does ;-)