The sensor lights up every time it measures noise - If used against a Supra LoRad cable you will be able to see that there is no noise.
The SUPRA AC sensor can be used to test radiation from sockets, cables, and various equipment. Because it can sense the noise, it can also be used to test the correct phase.
It can be held up to the C13 or C19 plug and should give sound from the side where the phase should be - Marked in red in the picture.
If you do not do it, you just have to turn the plug in the wall and then the phase should now be correct.