When I was young, I was facsinated by the arts of those sifus (teachers) who could easily submit a big guy with chin na technique.
It looks as if the rationale behind a chin na technique is simple: You use the strength of your arms / body to control a weak point (usually his joint) of your opponent (the "Weak Point Principle"). However, you will soon find that this is not the case, as you can not do it as smoothly as your sifu.
Your opponent will never cooperate with you as your training partner does. Your opponent will move his limbs and body instinctively, and it is quite natural, when he feels a pain in his joint. Once he modifies his posture (so as to relief his pain), the optimum position to control his weak point changes. Either you let your opponent go and you change to another technique. If not, you will find yourself end up locking horn with your opponent.
Ironically, the Weak Point Principle easily becomes another version of the Strong Man Principle.
The reality is: Whenever a sifu performs a chin na technique, he will first do some tricks to destroy the balance of his opponent. Once the opponent's balance is altered, he loses his ability to adjust his body position and is easily submitted to the chin na technique of the sifu. The best sifu can do the unbalance together with the chin na at the same time - the chin na technique is also the unbalance technique.
The key is to control your opponent's balance, not just his joint or weak point.