Thanks for the replies folks!
Wolf, I'd definitely recommend you try it. One thing I forgot to mention in my first post: she basically has to be on top for it to work. That way she can control the pace and take breaks whenever she wants. Which is a lot! I don't mind though, it's all part of the show.
I've just had a funny thought about it actually. Because she finds it so easy to orgasm and, conversely, so hard not to orgasm, she basically has to behave like a cliched teenage guy who's new to sex and is desperate not to shoot his load after only a few seconds! She has to do things like: think about other things, not focus on the pleasure and "sorry, I just need to stop for a second, I'm about to cum..."
Anyway, shall we move things on? Anyone got any thing from their own lives they want to ask about?