Loads and loads of rubber bands! LOL You just start with banding the main, and then for every layer down, you split the hair from the band in half and band each half to half the hair from the banded section next to it.
In theory it is very simple. And it looks ah-may-zing. But IMHO not the safest braid to be doing, so I'd only do it on a horse you REALLY trust. You can get caught up in it, and it can get in the way.
Edit; the more layers you do the higher the risk is - fewer layers means less traction on the mane so it's more likely to come undone if it gets caught on something. So relatively few layers (like the pic you posted) will be relatively safe but if you have a Friesian or something, and would have to do 10 layers, a running braid is safer. JMO of course.