I'm confused as to when this saves water. Are people turning the hot water on, then going to make a cup of coffee, checking hacker news, etc, and then returnin

I'm confused as to when this saves water. Are people turning the hot water on, then going to make a cup of coffee, checking hacker news, etc, and then returning some 10 minutes later to finally shower? Or is it turning the water off for half a second when you reach over to grab the shampoo bottle?