The reason why massage works is similar to why acupuncture works. It stimulates acupuncture points, relieving tension. A thorough massage will affect the acupuncture points regardless whether the masseur knows exactly where the points are, or not. Those who know where the points are often direct the massage specifically to the acupuncture points. Generally they use two fingers or a thumb on the acupuncture points, moving the fingers only slightly, alternating between superficial and deep pressure.
Massage can be good, or useless.
It depends on who is administering the massage. There are believers in the technique, and non-believers, depending on their experience with good massage therapists. It is mostly used on sport horses.