An NTI device is a small acrylic appliance worn at night on the front teeth. This allows complete movement of the lower jaw side to side and forward to backward. This not only allows the jaw to be in its most comfortable position, but also keeps the muscles of the head from exerting maximum force at night.
If you are wondering how it works, try this: Put your fingers on the temporalis muscles (the muscles on the side of your head at your temples). Take a wooden pencil and put it between your front teeth and bite. Your muscles can barely flex at all. Now try biting on the wooden pencil with your molars. The force is much greater. The NTI keeps your biting force off the molars and doesn't allow the muscles to work as hard.
"NTI" refers to the nocturnal inhibition of trigeminal nociception. The theory behind NTI treatment is based on the over stimulation of the 3rd branch of the trigeminal nerve. The 3rd branch of the trigeminal nerve provides sensations from everything below the cheekbone, including teeth, lips, gums, and the jaw joint.
Here are some more websites to learn more about the NTI:
Referenced from http://www.nti-tss.com/.