Manipulation of the spine or extremity is used to facilitate movement in particular areas that lack mobility or pliability.  The term "manipulation/adjustment" refers to the process in which the doctor applies a specific thrust (high velocity low amplitude force) to regions of the spine or area in which the doctor is attempting to restore motion.  

 

The "cracking" sound you hear during an adjustment does not indicate a bone moving in or out of place.  The sound you are hearing is gas being released from the synovial fluid in the  joint cavity.  Just about every movable (diarthrodial) joint in the body can be manipulated?  Meaning the shoulder, elbow, wrist, hip, knee, ankle, and any area with a joint cavity can receive an adjustment to help facilitate movement.