To use the Piano, the BBC micro:bit should be inserted firmly into the edge connector, either way round. If the Link Header is being used, the BBC micro:bit LED display should be facing the Piano keys.
Power is provided via a 5V micro USB connector, and the board then produces a regulated 3.3V supply which is fed into the 3V and GND connections to power the connected BBC micro:bit, removing the need to power the BBC micro:bit separately. This also powers the capacitive touch sensor IC and audio amplifier.
The potential uses for the :KLEF Piano for the BBC micro:bit don't have to be just musical in nature. You can use the piano keys to transmit from the micro:bit attached to the board to other micro:bits over radio. And if that other microbit happened to be inserted into a :MOVE mini... Piano remote control! You can even take control of the :MOVE minis ZIP LEDs! Of course, we 'tested' this extensively.