Share 'Back cracking????'

I've heard a few people talking about how they crack their backs before training, as it eases the spine up, a bit like cracking a knuckle, but I'm yet to discover how to do it myself. So, what's it all about? Does it make bending easier? How do you do it? I've tried twisting every which way but can't make mine make a sound. I'm fairly twisty in my midsection, not sure if that's got anything to…