To me. I have explained it to me in a way that makes sense to me, not as a circle, but as a deep dark lake with deep dark water and a humongous scary monster sitting in a deep dark cave smack in the middle of the lake's bottom. Anya magically transformed my quick flimsy sketch into the pretty diagram above, with the heroine (of course) jumping into said lake, about to meet her near death. 

Okay, let me step back a little. I have only started learning about this whole hero's concept recently, but I can already see it everywhere. Of course. It's the journey of life, in a way. And the journey of learning. We have to come close to death to learn anything, it seems. Stubborn idiots. Anyway. It does look like a boob in crosscut, doesn't it? The reason I made it this way is I can picture it easily in my head when I write and read, and if the simple three-act structure needs more subplots, then I can just break up one of the curves into more curves, like I did for the cave at the bottom. It could be simple, with no additional curve, or it could be more complex. This way any story can fit into this model. Precisely what I wanted to make sense of it.