Sears and Whirlpool dryers with the "SENSOR" had a set of sensors/wire tracers mounted on the stationary part of the back of the inside of the dryer, near the bottom. Each tracer was a circuit. As the wet clothes tumbled they brushed against each part of the two tracers, completing/closing an electronic circuit. When you initially started the dryer, the timer advance motor is OFF. The tracers/sensors are connected to a Sample & Hold circuit. As the clothes dry and are tumbling and hitting the sensors, the circuit gradually opens, since the wet clothes are less wet and are not completing the circuit anymore. When the Sample & Hold circuit is completely open, which means no more wetness in the clothes, the timer motor is programmed to turn on, which then starts to advance to the point of OFF. How long it takes to turn OFF depends on how far the user advances the timer, from damp to dry to more dry to very dry.
The Soft Heat feature worked in a couple of different fashions. If it the dryer was gas fired, there is a thermocouple/sensor on the exhaust port of the dryer and as the sensor feels the exhaust get hotter (as the clothes dry the exhaust temp increases), the sensor gradually closes the gas valve so the flame gets lower and lower as less heat is needed.
There were a couple of different systems if the dryer was electric. Some had two elements on the heater that cycled according to thermo discs in the air plenem. Some models only had one element on the heater and cycled the same way: on the exhaust air in the plenum. The gas Soft Heat dryers were a little better with the regulation of the heat. All in all its a great system and it worked very well........