But HOW is this possible? Concrete FFS!! It seems to go against the laws of physics,I know its not paranormal but its still rather amazing!
Like Sleepy said, the concrete is brittle. If you strike it correctly, it will snap. There is a formula, which I am sorry I can't write properly but you can see it
here but in plain words, kinetic energy (we called it power)= 1/2 X Mass X Velocity squared. This works not just for cars or machines, it works for your hand too. Since you get to square your velocity, if you can just go a little faster, you can develop more kinetic energy or power.
I don't know about Bikewer's comment; all of the stuff I was involved in was legit. No one was allowed to fool around with the breaking materials, no one was allowed to go up and slap the tiles first etc. In fact, the materials were tested first and if they seemed unsound they were discarded. We were not breaking stuff in order to impress the public. It was part of testing a student's abilities at proper technique. It sort of proved the person was learning correctly, without them having to do a lot of talking about it. As an instructor I taught my students how to do these breaks, in a sort of by-the-way type of thing. They only practiced hitting the air, or a padded target, until the test. I would not let them test unless I was fairly certain they would pass; breaking was a small part of the testing process.
Occasionally someone would miss a break, which means the board or concrete did not break. It's very embarrassing to go the hospital and tell them you hurt your hand by hitting a bunch of concrete on purpose.

Fortunately none of my students were ever hurt that badly, but I did know people who were.