Driving cools the radiator while still running the waterpump to avoid hot spots in the head. I'm not talking about racing around, just easy driving. Shutting it down is probably safe too.

AC triggers the fans thru a different part of the circuit, so when you jumper the switch you check the other part of the circuit.

Normally the gauge sits about 1/3 scale while driving. When stopped & idling it goes above 1/2 before the fan comes on. Its not the most accurate gauge, so some individual cars are different. Once you check this, it should be repeatable so next year it should switch in the same place on YOUR car.