Turning a valve behind one's head is not all that intuitive. When my wife and I were first learning to dive doubles and do valve drills she struggled with it. In one instance, she checked her isolator before our dive, just as we made a habit of doing, and found it was closed. Well, of course it was actually open, and she closed it before we dived. Fortunately, our dive was a practice dive on a shallow platform in a lake. It was a great lesson for both of us. After that experience, I got ahold of a "practice valve"--a cheap plastic water valve--and helped her learn which way to turn the valve no matter what orientation I held it in behind her head. That experience made me realize that even with practice, it may never become completely intuitive for everyone, and mistakes can be made.