It is the optimal T-man state. Shugart explains it very well in this article of his: http://www.T-Nation.com/tmagnum/readTopic.do?id=1749853
What makes you experience the flow? The gym of course but even Arnold of the 70’s probably didn’t experience it every single time he was training. When training, what things make it occur? Just simply realising that you pushed it harder than you could’ve ever imagined? I also have gotten it when I’ve been at an empty gym and have been able to fully concentrate in my own training. I don’t have personal experience but I believe having a good training partner will affect it. Franco and Arnold must’ve had a few!
Training aside, in what other areas of life do you experience it? Women and relationships? An excellent conversation perhaps? Right now I’m in kind of afterstate after writing intensly about something very close to me and the problem is that I can’t get out of it even if I wanted to. Writing this post doesn’t help it at all and going to sleep would be impossible. The gym I go to isn’t open 24/7, so I can’t train it off either.
Three best things in life might be to crush your enemies, see them driven before you and to hear the lamentation of their women but the feeling of flow comes fourth!