In cases with nerve impingement (a trapped nerve), mechanical decompression of the affected nerve has been noted, as measured by comparison of H-Reflex amplitudes pre and post manipulation (Floman et al). Mechanical decompression of a nerve helps improve the function of that nerve and reduces pain. Surgery for nerve pain (for example sciatica or carpal tunnel syndrome) is based upon the principle of nerve decompression.
Apart from the mechanical effects, influencing joint mobility and position, and adjacent tissues, manipulation also provides some interesting neurophysiological effects that help describe why manipulation can provide lasting correction of injuries, and is more than just a pain relieving tool.
More to follow shortly.