In a long article detailing the history of NAUI, NAUI founder Al Tillman talked about the major session in Houston in 1960 that led to the formation of the agency. In that session, people who had been instructing scuba before there were certifying agencies gathered in that city to work on techniques and help identify the standards that would be used for instruction. Tillman himself had learned to instruct at the Scripps Institute, where he learned so that he could use their methods to form the Los Angeles County program. In his description of the session, he talks about how surprised his group was to see some of the instructors harassing students by pulling off masks and shutting off tanks. It was clear to me in that article that those practices were done by a minority of instructors in those days, and many instructors, including Tillman, did not approve of it.
The reason I bring that up is that you often read people on ScubaBoard talking about how this was done in their instruction years ago, and they clearly believe it was a normal part of instruction back in the good old days when scuba instruction was so much better than it is today. The truth is, though, that if it happened to them, it was a choice by their instructors and not a normal and approved part of scuba instruction.
When I started technical instruction, it was done routinely to me and the rest of the students in my group. One day my buddy's air was shut off, and I donated my long hose to him and put my alternate in my mouth. Nothing. I realized that before he shut my buddy's air off, my instructor must have shut off the air to my alternate to simulate a left post roll off. (There had been no warning about this as a possibility.) I reached back and turned on the valve. No problem. A couple months later that instructor did the same thing with another pair of divers, but the diver with the left post roll off did not respond as I had. He panicked and bolted to the surface, thus yanking the donated hose out of the mouth of the OOA victim. This gave the instructor the opportunity to berate the one for his panicked bolt to the surface and the other for not having a firm grip on the donated hose. On the other hand, it could have just as easily given him the opportunity to deal with two dead students.
I teach technical diving now. Although I may well have become a somewhat better diver for having my air shut off on occasion, I will not ever shut the air off for one of my students. The marginal benefit it may (or may not) provide is not worth the risk.