They are a switch, for example, when there is power to the grid of a triode, current cant flow from the filament to the plate, when there is no power on the grid, power can flow from the filament to the plate, you see the filament boils the electrons off of it, and if there is power on the grid it blocks thoses electrons from getting to the plate, if there no power, the electrons flow from the filament to plate and make a current, they use them for signal amplification, but on Pt I will use them as a switch,
The only difference here is that instead of a filament, i use clone, and instead of electrons i use broken glass ;)
BTW the grid turns the tube on here, so its the opposite of a real life tube