In drizzle or showers, the serrated dial adjusts sensitivity and you should notice a difference in the wiping frequency based on the "quantity of moisture" present before triggering the wipers.

In a moderate rain or at higher speeds, it will switch to low. I have had instances in severe summer T-storms where it does kick into high. Once again, the threshold depends on vehicle speed and your sensitivity setting.

Mine's always set at the 2nd lowest.