Hello
From what I know, the spring pushes towards keeping flow closed. Pushing the closure disc by hand, there is some resistance to open by the force of the spring.
The pressure push from the water pump at high rpm does not force open the flow as pushing by hand does, is the opposite direction. Does not provide a 'relief' action.

The heat soaking/sensing/acting wax element is the rod at the top of the image or the cylinder surrounded by the spring ? (Flow direction is from bottom towards top)

When hot and open; is the spring force overcome by the (stronger?) wax actuator or the actuator never fights against the spring ?

 

You can observe the opening/closing action by putting the t-stat in a pot of water on the stove (no water flow from a pump). (I think today’s t-stats are bi- metallic and not wax, but I don’t know this for a fact). For example, a 180 stat will be fully closed below and fully open above 180 in this pot of water. In reality it will begin opening just below and be fully open a bit above. It will “modulate” (to use my term from the other thread) partially open. If you control the water temperature in your pot, you can keep the stat partly open. Installed in your car, it will modulate to keep the temperature at 180 (+/- a small amount). In normal conditions, once the engine is warmed up the stat will always be partly open. It will only become fully open if the conditions exceed the radiator’s ability to extract heat.

 

Fact.

 

I'll try to answer your question about what moves. The wax expands as it melts, pushing a small piston connected to the rod. As the rod extends, the copper pot is lowered, pulling the disc with it, opening the t-stat, compressing the spring. Hope this makes sense.

 

What Harvlr said is how it works.

FYI: The water pump doesn't create pressure, to open anything - it's not really even a pump, it just creates circulation to keep the coolant moving through the system - like a spoon in a coffee cup, really!