It's simple, when you turn on the ignition the carb heaters come on if the temperature is below the set value (don't remember exactly what temp). It doesn't use much juice and you don't have to wait even 30 seconds or so. You save more power make it easier to start than if it didn't come on. If it's warm enough to not be needed they don't come on.
Fortunately I rarely need to think about it. It rarely gets cold enough here and those are days I don't ride.
