A muscle does not contract as a single unit, like a rubber band, but a series of units called sarcomeres which can contract somewhat independently of each other. Trauma, repetitive use, or consistently poor posture can cause a small group of sarcomeres to stay contracted. This shortens the entire length of the muscle, causes dysfunction in movement, and can refer pain to different parts of the body. It also results in decreased blood flow to the tightened area. This contracted unit is called a "trigger point", or more colloquially, a "knot". 

TDN has been shown to be very effective in treating this type of muscular pain. During TDN an acupuncture style needle is inserted deeply into the muscle belly at the trigger point, causing the muscle to twitch or rapidly contract. In essence, it serves as a control+alt+delete for the tissue. This helps return the muscle to its natural length and restore normal movement and strength. TDN will also cause a local inflammatory response, which is GOOD. Inflammation is the body's natural signal to bring increased blood flow and healing cells to an area. When controlled, as it is with TDN, inflammation is a very helpful process. On a personal note, TDN is one of the few things that allowed me to recover fully from a 2yr long hamstring injury. It is an invaluable tool for both physical therapists and patients. 

TDN is not acupuncture, although the needles used are the same.