Found lots of useful info 're the way it operates, on 'the other side'.
Hope fully I am repeating this correctly, and broadly speaking, its a belt driven pump that takes water from the top of the engine, and pushes it through the heater matrix.
It's a bit like a/c, the revs increase as the switch engages a clutch on the pump that then circulates water, in this case.
The clutch automatically disengages if you drive off, or you can switch off manually of course
Those models with idle up switch, apparently did not have the pump, it just increased the revs.