This programmer uses a “UPDI+” connector which includes a secondary serial (debugging) port. This connector combines UPDI programming and serial communication. The serial lines can be switched off.

The 3.3 V and 5 V supply lines from the two modules are combined through Schottky diodes. This means that as long as one of the modules is connected to a USB port, power is available for the target. (The diodes chew off about 0.4 V which normally is unimportant.)

The power can be disconnected if the target runs on a separate supply or if you want to perform a power-cycle reset. (UPDI+ must also be switched off or the processor could be powered through the clamping diodes of the serial input pin!)