Where does joint manipulation fit in if people can "pop" lax joints on their own and have a friend "pop" stiff joints for them? On one hand "popping" a joint does provide temporary pain relief and an increased range of motion, but on the other if performed incorrectly it may result in further injury.
"Popping" a joint releases endorphins and stimulates nerve receptors that detect motion (mechanoreceptors). These endorphins cause us to feel pleasure, they are released during exercise, improve our mood, and make us feel better overall. Mechanoreceptor stimulation overrides nerve receptors that detect pain (nociceptors). When this process occurs your pain level should decrease and you should have an increased range of motion in the "popped" joint. The issue is that this result is short lived when joints are treated incorrectly.
It's extremely important to distinguish between stiff and lax joints during the "popping", i.e. manipulation, process so that sprain/strains and further pain & stiffness doesn't occur.
1) Stiff joints (hypomobile) - Are restricted beyond their normal range of motion. Typically this is a result of scar tissue formation in muscle, tendon, or ligament tissue. Scar tissue is not as elastic as muscle, tendon, or ligament tissue and therefore limits motion making it feel stiff. The stiff joint is the ideal joint to manipulate increasing motion and directing an increased blood flow to the scar tissue allow for further healing and ultimate scar tissue removal and muscle restoration.
2) Lax joints (hypermobile) - Commonly surround stiff joints and move extra to make up for lost motion by becoming lax and unstable. When lax they should avoid "popping" because of their instability and being susceptible to sprain/strains. These joints should focus on strengthening and stabilizing.
Ideally every joint should be stable and strong enough to support its job. When these issues arise it puts the body at an increased risk of injury and chronic issues down the road. Professional treatment should be sought to target and manipulate the stiff joints allowing for the lax to re-stabilize. If properly treated many chronic flare-ups can be limited and even eliminated.