Let's say Professor Smith wanted to use Understoodit. She would start by creating an account and choosing the screen name 'smith'. Professor Smith would then tell her students to go to understoodit.com/smith on their laptop, tablet or smartphone. When students are confused they would click the 'confused' button, and when they understand they'd click 'understood', and in real-time Professor Smith can see the percentage of students that understand. It's that simple.
Professor Smith's students only need to create accounts if she wishes to keep track of their progress or answers. This means that Professor Smith can use Understoodit at conference presentations, even if her audience members don't have Understoodit.com accounts.