What does the cool button on a blow dryer do?

The cool button on a hairdryer is mainly used when blowdrying the hair with specific brushes. Inside each hair are things called salt linkages and hydrogen bonds which when heated can be molded into a different shape or form (such as blowdrying hair from straight to bouncy curl) to ensure the shape stays set when the blowdry is complete it is essential to use the curl button on each section you have used the brush on to cool down the salt linkages and hydrogen bonds so they fixate and the hairs shape stays put :)