I think I would have to see a youtube video of this to really believe it. Although I have tried on numerous occasions to see if it was remotely possible, every slab I have cracked has either ended up splintering all over the cracked along the side as Bomber Bob described. And even if I could pop one corner with even a slight crack, the second post inevitably destroys the slab every time (or at least made it extremely "tamper evident").
-J.
Perhaps if you had read his posting you would comprehend the point he was making; Brought the slab in "directly from the cold"
The colder the temperature the greater the contraction of the material, so what beaumonts is reporting is completely plausible.