As far as I know, CSS recommends "not hanging off TOO far" ONLY because it is a rare rider that can hang off that much without putting some pressure on the bars or losing the stability of their position. I'm pretty confident that if you could show that you could hang off more than a half-cheek and still be solidly locked on, feel comfortable, and have NO weight on the bars, your coach would be totally ok with it.
More commonly we see riders trying to hang their butt off so far that they have to hold onto the inside bar, which has a big negative impact on the bike's handling - so we ask the rider to hang off less, get the lower body very stable on the bike, and get rid of that unwanted bar pressure, which generally results in the bike handling more predictably and a much happier rider.
Now that makes sense to me :-D
Thanks
