An unsafe device which uses your body as part of the test circuit.
This device is a poor quality screwdriver with a neon lamp inside the plastic handle.
When in use, the circuit is:
live item - screwdriver blade - small resistor - neon lamp - spring - YOUR FINGER
The safety of the device relies entirely on the small carbon resistor inside the handle. If this gets wet, dirty or just fails, then your finger will be receivng the full mains voltage on it.
If kept in a totally dry environment and used properly, this item might be safe. Unfortunately in the real world this won't happen - and this example has a large hole in the end so that water and dirt can easily enter the device.
While these things do illuminate in certain circumstances, they can also illuminate when there is no power at all, or fail to do so despite lethal voltage being present.
The neon is very difficult to see in bright light or outside, and the brightness of the neon depends on how good the connection to Earth is - this connection being via your body.
The only thing holding the contents together is the spring - should any of the internal parts be slightly dislodged, the device will not work.
Another fail. The blade is made from a rather soft metal which will bend and distort easily.
The tip is flared - useless for terminal screws as the flared part often won't fit into recessed screws.
This example has a bend in it, despite being new and unused.
Hopeless. Even if this does illuminate when mains is present, this tells you very little. A common problem is the neutral connection to a device being broken or loose. This tester will still illuminate, indicating power is present, telling you nothing about the real problem.
For poking about inside possibly live equipment, this tool is a disaster. Note the excessive length of exposed metal blade - when in contact with a live terminal, the whole blade is live, and could easily slip and short against another terminal.