Stop in for a cup of coffee

So I have an electrical question that I’ve not been able to get answered via a Google search.


Summit and others sell these add on idiot lights(warning lights) for after market and factory gauges. How do they work? Like how does the lights know when to turn on if say the oil pressure dips too low or the voltmeter drops etc?
The sender is a off-on switch. (you need a separate sender for a light) Usually comes on at just a couple of pounds.