Like all complex life, humans are amazing self organizing/self maintaining organisms, and the best thing is our bodies have an intelligence that works without us having to exert any direct control. This is good because all the factors governing function, repair and balance are too complex for us to have to "do it" ourselves. The end of a bone will usually match the joint capsule it fits into. If the bone becomes distracted from the joint capsule it might not go back in if the ligaments are coiled up and won't let it. Distracting the bone and holding it in a different position can allow the ligaments to uncoil and the bone may go back into the capsule as if it "wants" to go back in, because the end of the bone and the joint are shaped to fit together.
In like fashion, many things that a Rolfer does that seems like an "adjustment" is really the Rolfer helping the structure to find its natural place again. The could mean anything from vertebrae to muscle tissue. The body has an intelligence that strives for balance, but sometimes needs assistance to allow the intelligence to find what it needs to do. Some of this is based on shape and structure, a square peg many need help finding the square hole it fits into. The Rolfer's real job is to give the client's body input their body will re-organise itself around, and in that way, the Rolfer does not do it, it is up to the intelligence of the body to do it. Muscle tissue and muscle tension function in the same way. Muscle tension is governed by the body's own intelligence, and no amount of stretching and massage will change that if a group of muscles need to be tense for the person to function in their daily life. It is the Rolfer's job to help the body find balance and ease, so that unnecessary tension can be minimized.