NOT Gates are used to make logic gates. By default, the output is inverted, so to turn on the output the input that is connected to it must be off. This allows it to be used as a flicker switch because it turns off then on because of the inverted output.
The analog value is equal to 100% subtracted by the analog value of the input. Also, other gates can be inverted as well, and the NOT Gate's output can be manually set to uninverted using the tweak tool of the Popit.