Seek the god of physics!

If the switch is off, there is no current at point A and point B.

Similarly, the live wire and the neutral wire are connected in series, and the current comes in from the live wire, and the switch is connected to the live wire, thus ensuring that there can be a potential of 220v before the switch is used. The back emf is 0v. No loop is formed, and there is no current in the whole circuit.

At this time, it is safe to touch the place behind the switch. Never touch the place in front of the switch barefoot, otherwise a circuit with current and electric shock will be formed immediately. If the current on the insulator is small, it is relatively safe