The explanation
@JohnW2 seems good, however my non contact volt tester seen here
is detecting volts with my hand no where near it. You can see all four bars showing, and it was beeping and the red LED flashing rapidly. I know the device uses the hall effect to measure DC current, and on DC it needs zeroing, and slight movement can alter reading, so although measures in 1 mA increments, I would only take it as an indication, the AC current range however is far more stable, again 1 mA increments.
I really have no idea what is in the jaws, or what voltage it will detect, never tried on low voltage DC or tested on low voltage AC, would be interesting to try at work on reduced low voltage and see if it works.
I had thought the volt pens used a reed switch, however not sure what the sensor is, and in all cases not sure what they will do with DC, I suppose easy enough to test, I could hold close to the solar panel leads, they are around 250 volt DC.
But the volt stick is non contact, the neon screw driver makes contact the metal clip needs to be touched with your hand, and the screwdriver tip needs to touch line, and there is a resistor to limit the current, the worry is if the resistor is shorted out with dirt etc, when stored in a tool box.
I think the neon tester is a very good second string to ones bow, for detecting borrowed neutrals, the clamp on should detect current if clamped around a neutral wire carrying current, but often not room enough to get the jaws in, seeing the neon in the screwdriver light when disconnecting a neutral can save one from a nasty shock.
But my meter does allow one to use leads as well, so the non contact voltage is good for testing a FCU before removing the 3.5 mm screws, but once the FCU is dangling on wires I would retest with leads to show really dead. Did not fit leads in today's test as they may act as aerials, and wanted to see if it can detect voltage without having the leads in place.
I don't know how the testers work, however I do know if a pen detector is dropped, they can stop working, so testing it works before and after is important, I have seen them rubbed on clothing to test, but I am weary as to if this test is any good.