Great idea to post this, thanks!
I turned it into a sticky with a kind of level tag and a slightly more clear title, so don't think your post is gone.
I love this exercise because it takes away all the 'dominance' pressure from backing up. Most horses dislike backing up because it's most of the time taught by blocking the forwards motion, with backing up as only option. I taught Blacky backing up tail to hand because he needed to strengthen one of his hindlegs more with backing up and sideways movements, and he was getting more and more fed up with backing up with me next or in front of him.
I did it just as you did: I stood behind him and snapped my fingers (our cue to touch my hand), and instead of turning round and walking to me (what I had meant), Blacky stood still for a second, and then backed up towards me, untill his tail touched my hand. He sure earned a lot of treats with that!
The good thing is that he not only invented this himself (I had never seen anyone do it before either, so it truly is his invention), but also that he loves to do this. He will back up 10 to 20 meters, with me telling him how good he is all the way of course.
Sjors invented it the same way recently. Sometimes when I stand further to the side (for example when lungeing) and ask him to come to me, he turns his hindquarters towards me and walks backwards to me.
Another nice variation is something to do when the backing up to your hand goes really well: instead of posting yourself behind him, you can then start to stand more behind-to the side, and see if he discovers that he can not only back up straight, but also in angles and curves.