With respect to the needle thing, If you shut down the engine, all needles needing electrical input should go to zero, as you cut energy.
With regards to the temperature, if you shut down an overheated engine, you severely risk permanent damage. If you stop the engine, the water pump stops working. Then, as the water is not moving, and the chambers are still very hot, it starts boiling. Water steam does not cool anymore so, if you are lucky, you cook the block joint, if you are not, you bent the head, maybe the block itself. You could limp home, but you would be leaking water and cylinder compression. Thus, less performance and, eventually, a dead engine.
The right way to cool an engine is to iddle it. NEVER stop it. You should cook your block joint (at least) if you stop an overheated engine.
Cheers,
Pepe