This thing - upon successful calibration - will detect strong static fields, strong VF/ULF fields, RF fields, visible light and ionizing radiation - moderate amounts of UV radiation, and certain milirentgens (more than 50mRs) of gamma radiation (it can work like a less sensitive Geiger counter). No faerie magick involved - the detector itself is only a small Neon glow lamp (NE-2), tuned just below its striking voltage.
The circuit is powered from two 9V batteries in series, input voltage slapped to about 16V. The only major drawback is, after powering on, you have to calibrate it away from strong electromagnetic fields (it takes 4-5 seconds), and for very sensitive operation (e.g. detecting 230V mains cables in the video), it detunes after some seconds. For strong RF fields (or ionizing radiation) or strong light sources, it isn't necessary.
In the video, the device is calibrated and then placed near a live mains cable, a light switch, then exposed to an incadescent 60W light bulb and a ultraviolet light source (four Philips 15W mercury tubes).