On or off.
Unless you have special sparkless mercury switches where the contacts are made in a sealed glass tube, there always is a small electric arc on making and breaking the circuit. Whether it’s enough to ignite the gas/air mixture is another question. Sometimes yes, sometimes no.
I haven’t seen a mercury (light) switch on the market for years. I have one or two of the tubes in an old parts drawer. Of course, old installations still in use - what’s to fail?