I have malocclusion III (or known as underbite for us human), and my pet peeve is my jaws popped, locked, and cracking from time to time. I sometimes have pain on the side of my face & temple area, I figured I was going to catch cold or something.
Before starting my first physiotherapy session with some massage & showing me some jaw exercises I can do at home, Mr. De Jong pointed out that I have TMJ problem.

What the heck is TMJ??

Below is what I can gather from Internet sites and lurking at orthognaticsurgery mailing list:
Temporomandibular joints are located in front of each ear. These are the points where lower jaw connects to the skull. It moves laterally, rotates, in and out of joint. In ideal world, there is a balanced closure and full contact of all teeth at once. In my case, with my misaligned teeth the muscles become spasms as the jaw tend to deviated to one side. This in turn constricted the blood flows which can cause a muscle cramp, and even headache!

How about the popping, locking and cracking of the jaw? The joint rests in a socket lined with pad, it prevent both bone from wearing each other. When the jaw moves, the pad moves along. However, if the pad moves way out of its position, the jaw locked up and it isn’t possible to close the jaw. The popping and cracking also happen if the pad is out of its position but the jaw is able to close.

Interesting fact: most TMJ sufferers are women. It has something to do with the way women handle stress. They intend to internalize their stress. Usually people tend to clench or grind their teeth more during stress. It pulls jaw muscle to one direction and increases the pressure on TMJ.

Some people got TMJ disorder due to injury on their jaw. So, there are also a lot of factors contributing to TMJ.

Now, back to my story. It turns out, my jaw has limited movement, more so when I have to move it backward. I also have less mouth opening then normal which supposed to be around 45mm, or something. In preparation for the surgery I have to do these excersizes so my mouth can open wider, and more supple.