This is not a new invention. A capacitance sensor connected to the saw blade, can sense the change in capacitance from human body. If this happens, a small explosion on an inside mechanism releases with tremendous speed a specially designed metal plate, which is able to instantly absorb the energy from the the saw blade and stop it. Of course many mechanical parts wave bye bye. But the point is that you still got all 5 fingers on your hand, and your hand on your arm.
This invention did not became very popular, despite the fact that the inventor used a lot of sausages to demonstrate it. So now, he decided to promote it by endangering himself! He puts his finger on the rotating saw, in front of a high speed camera, to prove that the system works! If it fails, he gets a slight cut... The invention is called SawStop.
saw stop is many years in production, but has not yet become that popular. As i work in wood working machines, i kinda know what safety rules are applied to them. When i first saw this system, i thought that in a couple of years, it would become a standard safety equipment (like safety mats on CNCs), and that every new sold machine would have this (or a similar one). But i was totally wrong. Nevertheless, fewer and fewer carpenters have less than 5 fingers on each hand, as machines have become generally more reliable/automatic/precise and safe.