In either case you can have a problem. You simply cannot tell whether a muscle is strong or weak by whether it's contracted or not. Chiropractors adjust the joint to break the splinting action. Physical therapists attempt to strengthen the longer muscle. This worsens the imbalance.
Problems can arise in all three cases.
If a chiropractor adjusts your neck and your neck muscles don't lengthen in the process your muscles can spasm and traumatize the joint capsule even further. The other two disciplines applied in the normal fashion can also make the problem worse. What David Scott Lynn does is simplicity itself, at least when you first develop the ability to comprehend it. What he does is look for contractions in the muscle that need to be released. You look by probing with your fingers.