That switch has long been a feature of guitar amps as a misguided 'solution' to humming. What it does, is opens ('lifts') the connection to Protective Earth (called Safety Ground in North America).

In other words, it addresses the problem of ground loop noise by removing the ground connection of your amp. This may reduce the hum, but it will definiteley put your life at risk, if you are plugged into bad AC wiring.

Set it to connect your chassis to Protective Earth, and then leave it there! Safety Codes on both sides of the pond have been updated to make this switch illegal for new manufacture.