thermostat operation

In his book he mentions Use ECT data to check for a stuck open thermostat. test drive at a constant speed over twenty five mph while watching ECT temperature. 

My question is once a car reaches 210-230 degrees, it should never get under 210 while driving at a constant 25+mph speed. Is that correct? if so, why is this. What's the theory of operation Danner tries to explain in his Chapter 6 "Thermistors" pages 10-11, 16-17 video.

  once a car reaches 210-230 degrees, it should never get under 210 while driving at a constant 25+mph speed. Is that correct?

The temperature range depends on the specs of the thermostat.

if so, why is this. What's the theory of operation Danner tries to explain in his Chapter 6 "Thermistors" pages 10-11, 16-17 video.
 

As I'm sure you know, when the engine coolant warms up to a certain temperature, the thermostat opens and the engine cools. When the engine coolant cools down to a certain temperature, the thermostat closes and the engine temperature begins to rise, again.  Operating in this way keeps the engine coolant within the temp specs of the thermostat. If the coolant cools down to the closing point, but the thermostat does not close, coolant flow will keep cooling the engine, and the temperature will drop below that of the closing specification of the thermostat. So, if you see the engine coolant temperature fall below that of the T-stat spec, you can suspect a stuck open thermostat.
 

Another indication of a stuck open t-stat is an engine that takes too long to warm up, and/or never reaches operating temperature.