One of the main reasons for the pause during washing/rinsing is to generate the higher pressure at startup that the upper wash arm needs to clean dishes/cups/glasses positioned in the corners of the upper racks. Some folks actually have thought that there is something wrong with their machine because it "just stops in the middle of the cycle"!! Sensing happens while the machine is running as it senses the pressure of the water moving through the dishwasher's pump chamber...as pressure drops it indicates a "heavy soil" condition, which will trigger a purge, and if needed, additional heat or rinsing. If pressure remains fairly steady, no purge is triggered. This is in contrast to the purely optical sensors used on other brands, such as GE's extra-clean sensor that can "see" the quality of the water moving through the pump and add time, heat, rinsing to the cycle. The higher end Kenmore Elites, Whirlpools and KitchenAids can also measure the size of the load in addition to soil and fill, set wash/rinse time, temp and cycle length accordingly.
So what happens if the sensor(s) malfunction? Does the machine default to a pre-programmed cycle or just give you back dirty dishes?