The Predators clearly had superior strength, speed, endurance and agility. The Predator could lift up Arnold with one arm and jump from tree to tree etc.  

I would surmise that they have the strength to do this BUT I agree; being able to rip out the spine "cleanly" does seem unlikely.  And it probably would come out in pieces (not as a whole unit)  

Maybe they break all the ribs first and make some cuts..? 
 
And no, I've never seen an explanation of how they do it. Hey, can we ask Mythbusters to test this out?