I do not know that much about tricks, as I never studied to do them. I do understand that both the "bed of nails" and the "sledgehammer" tricks take advantage of increased surface area to dissipate the shock of strikes. Ask someone to take the sledgehammer strike to the chest without the concrete block, or sit on a bed of one nail.

As for a good demo, how about the instructor sparring everyone there?
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"In case you ever wondered what it's like to be knocked out, it's like waking up from a nightmare only to discover it wasn't a dream." -Forrest Griffin