bypass hose

One more time;
the thermostat sets the MINIMUM coolant temp.
The efficiency of the system sets the max.

The minimum is supposed to be around the rating of your stat. When you start your cold engine, minute by minute, the temp increases, and eventually gets to the set-point. Then the stat begins to open. When it does, often the cold water from the rad will cause a brief hiccup in the stat and if your gauge is fast enough, you can catch it.
But the temperature will continue to go past the set-point, and if your system is down on efficiency, especially at idle, the temp will continue to rise until it finds equilibrium, even tho the stat is wide open.
So by now, you are itching to go so you put it into gear. This slows the engine reducing efficiency, cuz you are still not moving; and so, the coolant temp continues to rise. Finally you begin motoring away, the rpm comes up, the fan is spinning and the pump is pumping; efficiency is on the rise and the temp falls TOWARDS the minimum. If it should happen that your system is exceptionally efficient,and the coolant temp falls below the minimum, then the stat will begin close. Few systems will do that unless your stat set-point is quite high.
Any time the stat is closed, coolant will be forced back into the pump for a second trip thru the block. Some of it can be diverted into the heating system.
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In my engine the bypass is severely restricted, and the cab-heater is my bypass. This is my back-up radiator in case the main system can't keep up.
After I installed the Milodon hi-flo pump/ thermostatically-controlled, 7-blade all-steel, factory fan; the cab-heater safety back-up, has never been needed; so I put valves in the lines, and shut the heater off for the summer. Now the restricted bypass, makes sure my alloy heads come up to temp in a reasonable period of time.
But the fail-safe is still there, all I have to do is open the valves.
I run a hi-Flo Milodon stat rated at 195. The coolant runs a rock-solid 205 to 207, measured at the stat-house by IR gun; no matter what the circumstances, cuz that Ford pick-up fan-clutch is flippin' wonder.