All that's inside them screwdrivers is a neon lamp and a resistor.
When the blade touches a live wire, a current flows through the screwdriver blade, to the neon indicator, the resistor, the end cap of the screwdriver and through your body, eventually to earth, usually through the soles of your shoes.
The value of the resistor is chosen so enough current flows to light the indicator, but not enough to give you a nasty shock .... in theory!....
But.....
If the neon fails, they don't light up
If the resistor fails open circuit, they don't light up
If you aren't holding the end cap or clip, they don't light up
If you aren't in contact with a reasonable earth, they don't light up.
If the innards start to oxidise, they might not light up.
If you are wearing the wrong shoes, they might not light up.
If you are working from a wooden step ladder, they might not light up.
If you assume a cable is dead because the neon doesn't light, you may soon find out otherwise, maybe by suddenly finding yourself leaping backwards from the top of a pair of steps if you are lucky.
If you are not so lucky, you may never know what hit you.
If the resistor fails short circuit, there's nothing limiting the current, so when you use it, you light up instead!
If they get damp, the moisture can short-circuit the innards, again causing you light up.
If they get damp, then dry out again, there may be an invisible track of salts deposited, shorting out the innards, so again you light up instead of the neon.
That's just some of their good points.
They are no substitute for proper test equipment, no matter what the salesmen say. They may even be more dangerous than not testing at all, as you may proceed with a job with misplaced confidence after a false negative test.
