The correct name for a sound made during a chiropractic adjustment is a ‘cavitation’. It was once thought that this sound was produced due to an oxygen bubble being popped inside the joint (and therefore the myth of it causing arthritis!), but recently this phenomenon has been recorded via MRI and analysed in depth to show the true cause of a cavitation.
Anatomy - The most common joint in the body is called a synovial joint. A synovial joint has a capsule surrounding it which contains a fluid called synovial fluid, this contains a mixture of fluid and gases which together help protect the joint.
Science lesson - Boyle’s law states that when you increase volume of a fluid that's contained (i.e. move the 2 joints apart) the pressure decreases proportionally. This pressure decrease within the joint causes some of the liquid to turn into gas, (mainly carbon dioxide) very rapidly which causes the cavitation that you hear.
To see a video of this in action click the link below!
https://www.youtube.com/watch?v=aJLU-4M-hdE
So, is hearing the cavitation during adjustments important?
The short answer is no. As satisfying as it sounds during an adjustment the evidence doesn't show any difference to joint movement after an adjustment that has cavitated and one that hasn't. Also some joints will need to be adjusted in 2 directions, therefore once the bubble has formed on the first adjustment to that specific joint causing us to hear the cavitation, it won't be possible to get a cavitation again in that joint for about 20-30 minutes, so if a second adjustment is needed to the same joint it is unlikely to 'click' again.
If you have any other questions about this please don't hesitate to ask!