I got told the bar is there in case your clothing gets caught up in a Brach and starts to pull you in. Never pushed the bar so I am not sure if it does any cool stunts like a backflip.In the late 60s, some chippers had an emergency stop bar, but it was discontinued because of accidental trips.
Furthermore, stopping the drum (which spins at 2800 RPM) quick enough to prevent an amputation would create a recoil that would tip the machine backwards possibly on top of the purported victim.