In reply to McBendy:
Problem is when you fall off the line you have to re-mount from underneath which is really hard to do.
Once you have the technique dialled you can save yourself from some pretty serious wobbles which means you always look a bit out of control when slack lining. It's also easier to balance with one foot off, the other leg acts as a massive counter balance much more than the raised arms do.
When you're high lining the line is always tighter then a normal slack-line, you would also have a double webbing system (tubular webbing inside another tubular webbing) for extra strength and safety. If you are wearing a harness and safety leash you never really want to fall on the line and would always try to catch the line with your hands and behind one knee. A tighter line is also harder to balance on as the line wobbles much faster, the dynamics of the wobble also change with every step (closer to the anchor is tighter than in the middle).
Massive respect to the guy, would be a huge head game to do thateven with a safety line set up.