The heater core loop is open/running whenever the engine is running. It returns to the engine through the thermostat housing (that's what you always hear generically referred to as the thermostat bypass) - it may be that the thermostat sensing element gets directly bathed by that flow until the thermostat opens, or maybe all the time - I forget.With this image, I am at a loss to figure out how coolant can move from top to bottom as Peva said. If all the hot coolant was flowing from the top hose and then through the radiator, it would not be able to reach the thermal sensor in the t-stat.
I tell you what - the guy who could explain that exact thing is FireM (Bob Day). He has posted threads on it more than once on the 300M Enthusiasts Club forums, and maybe even once or twice here. I had the same question as you, and he set me straight on it, but it's been a few years.