Doug will probably respond, but I'm pretty sure the method he teaches is to learn to flick the flame away from the tube before it gets too hot. This is the method I settled upon decades ago, and it even works with a torch that's too big. The trick in any event is to notice that the color is increasing while you are concentrating on everything else. Maybe I'll make a video.
A common problem with teaching something you know really well is that there are many things that you don't know you are doing. I think this is one instance where Mr. Brodie is falling into this trap.