I've seen more than a few demos like that back in my kuhrotty daze, and even did some of them. But at least I used actual store-bought concrete slabs from Home Despot. You know -- the ones that break if you even so much as accidentally shuffle them when you load them into your truck.
Once, I had to shuto a watermelon because my boss made me do it at the company annual meeting.
Seriously, I agree with Mike. Those slabs were probably made for the demo, and I'm guessing they are mostly Portland cement or Quickcrete prepared so it was nice and crumbly. The dude didn't use spacers, which is a common trick that allows the "breaker" to individually break each stacked slab as his hand passes through. But there may have been a crack or score pre-made in the center of each slab before the demo.
Though this was an obvious fraud (BTW, the flames are just lighter fluid, not hot or intense enough to do more than scorch any hairs on the guy's hand.), legitimate (i.e. not using rigged items) external-skills breaking is just a demo of the person's ability to use good form -- a solid path through his bones to the ground so the power is passed through without interruption; directing his force through the smallest possible surface area of his hand (usually a point on the palm-heel, but sometimes a shuto/knife-edge); and his ability to accelerate and to strike the objects at the peak of acceleration, and to follow through.
That's really it. A tornado can drive a thin straw through a tree trunk without breaking the straw. It doesn't take that much acceleration and velocity for a human to accelerate through a stack of concrete slabs.
And if you have nice stack of kiln-dried pine wood, with a nice horizontal grain, all the better.