When the spell is pronounced correctly and understood by the Pi, it tells an Arduino to ‘shoot’ the spell out of the wand via an infrared LED. If the infrared receiver attached to the opponent recognises the spell, it sets off the TENS machine to deliver an electric current to the appropriate body part. Expelliarmus, for example, sets off the TENS connected to the arm, while calling out a successful Petrificus Totalus renders the opponent near immobilised as every pad is activated. For a moment’s rest, calling out “Protego” toward your own infrared receiver offers a few moments of protection against all spells aimed in your direction. Phew.