carbon is a poor conductor of heat so they should not get burned if the coals are done correctly - something was done incorrectly. It’s supposed to be a sham, but it turned out to be a cluster.
The power from within was unleashed - they got what they paid for.
The Mythbusters did a “firewalk” demonstration. Jamie did OK, but Adam didn’t quite do it right. At the end he was hopping around in some pain.