As a Chiropractor this is one of the most common questions I am asked. Firstly you crack eggs, not spines. Chiropractors are university qualified, government regulated, primary health care practitioners highly trained in diagnosing neuro, orthopaedic and mechanical conditions of the spine and body.
What is that cracking sound?
In between each of the joints of your spine is a “lubricant” type fluid called synovial fluid. Synovial fluid is full of dissolved gas and when the joints of the spine are opened or corrected in a rapid movement it creates a vacuum in the joint. This vacuum sucks the gas out of the lubricant, forming the lubricant into foam. The release of gas is what creates the noise of the pop or click. If you do not hear a noise when getting adjusted, it doesn’t mean it hasn’t worked, it means that there was no sound from the gas escaping the joint.
There isn’t an actual pop or click of your spine as many people may like to believe, the audible sound is called a cavitation and it is created by nitrogen trying to fill the space created by the vacuum effect of the spinal joints opening. The same phenomenon happens when you open a bottle of champagne, when you take the cork out you hear the pop of the gas leaving the bottle.
There are many different types of Chiropractic techniques, and not all of them produce cavitation or audible noise of the joints. Some techniques use very low force and no cavitation noise at all. Chiropractic adjustments are not all about getting the noise or cracking your spine.
A Chiropractic adjustment may be conducted to correct the affected spinal joints, the Chiropractor may use their hands and or an adjusting instrument. You may hear a pop or a click noise, cavitation, rest assured that this is perfectly normal.
A cavitation is just gas releasing from the joint rapidly. Recent MRI studies have explained why this phenomenon occurs, to put it simply, when gas is released from the joints at a rapid speed like in a chiropractic adjustment, it is like a small explosion of gas, this rapid release and change in joint pressure causes the noise of the cavitation. 
