When a manipulative therapist (such as a chiropractor) treats the spine, the aim is to apply a short and sharp thrust in a direction that will separate a joint’s two opposing surfaces, pushing the joint apart.
As the joint rapidly separates, the synovial fluid gets caught in-between the two joint surfaces due to the sudden decrease in pressure. This is called a viscoelastic adhesion and you might have come across a similar situation when lifting a glass off of a wet tabletop. The water can get caught between the two objects and stick the glass to the surface.
With the joint surfaces continuing to separate, the negative pressure becomes too much for the synovial fluid, giving way to a process known as tribonucleation. This event occurs when the pressure gets low enough that a near-vacuum bubble forms and gases dissolved within the synovial fluid begin to evaporate into it.
The new gas bubble is easier to stretch apart that the semi-solid viscous synovial fluid, resulting in the joints being able to separate more rapidly. As it grows, the pressure in the bubble continues to decrease and gases from the surrounding synovial fluid continue to infiltrate it.
Next, the synovial fluid from the surrounding, unaffected parts of the joint begins to move in towards the cavitation bubble. The immense pressure differences cause the sides of the bubble to slam into each other with great force. It is this collision that causes the cracking sound we all know (and some love) . The whole process takes milliseconds and the exact mechanics of it is still debated to this day.