An adjustment, also known as a “manipulation,” is a controlled force that it directed to a joint that has lost a normal degree of motion. A manual adjustment is one that is done by hand. Often, a “popping” or “cracking” noise can be heard from the moving joints, though this is not always present and does not indicate whether or not the adjustment was “successful” or not.
Each part (joint) of the spine is responsible for a certain degree of motion, which allows you to move (rotate, bend forward and backward etc.). Some times, due to fascial adhesions, scar tissue build up, repetitive activities, muscle imbalances etc., certain parts of the spine will not move as much as it is supposed to move. When this happens, the other joints in the spine have to “pick up the slack” (because the movement has to come from somewhere!), and have to move a little bit more, creating an imbalance. Too much movement causes unnecessary wear and stress on the joints, while too little movement causes stagnation of joint fluids and improper lubrication of joints, both of which can lead to premature degeneration. The adjustment seeks to normalize the motion through the spine (and in the arms and legs too!).
Besides cases of hypersensitivity or pathological conditions, this adjustment should not be painful. In fact, it is fairly uncommon that someone dislikes the manual adjustment, and some people even experience euphoric feelings after being adjusted. There are many ways to manually (by hand) adjust, so if you prefer one adjustment over another, changes can be made to make you more comfortable.