It works through a surprisingly simple mechanism on the left side of the mouse. (digitaltrends.com)
Finally, on the left - hand side of the mouse there's two thin buttons that fall neatly under your thumb, and a larger circular button that lies just at the tip. (wolfsgamingblog.com)
Technically, you can only use nine unless you've trained your ring finger to access hard - to reach buttons on the other side of the mouse. (digitaltrends.com)