Tank yoo Paul!
Some scientists wanted to learn more about knuckle cracking, so they actually stuck a sensitive microphone onto a finger. They found that there wasn't just one single sound when you cracked a finger joint - there were actually two separate sounds. The joint space is the space between the bones. There is a liquid in this space, and there are ligaments on each side, holding the bones together. As you pull on the joint, you first drop the pressure in the joint space - and the ligaments get sucked in. Once this pressure gets low enough, a bubble pops into existence - making a popping sound, which is the first of the two sounds.
Now this bubble has a certain size - on average, about 15% of the now-bigger joint space. Because the joint space suddenly has a bubble in it, the liquid, just as suddenly, pushes on the ligaments - snapping them back to their original position. This snapping back of the ligaments is the second sound.
The energy set loose inside the joint is only about 7% of what you need to damage the cartilage. But if you crack your knuckles often enough, you can end up with swollen ligaments.
Another study looked at 300 people who had been cracking knuckle joints for 35 years. They had slightly swollen joints (which is no big deal). But the real surprise was that their hands were weaker - their grip strength was one quarter as strong as it should been!
Tank yoo Paul!