Structural Integration is is a codified series of soft tissue manipulation that organizes soft tissue relationships with the objective of manually realigning the body structurally. It was developed by Dr Rolf (hence the naming of the technique after her) in the early to mid 1950's.
She discovered that bound up fascia often restricts opposing muscles from functioning independently from each other. She could separate the bound up fascia manually by deeply separating the fibers. Exactly what shaping of pushing and how deep was necessary to free up this bound-up fascia obviously required extensive knowledge of living anatomy - and extensive training. Without this knowledge, pushing deeply can leave bruises.
As Ida Rolf worked with people, she discovered a link between muscle tension and suppressed emotion. Evidently, when tension is released, some people experience "flashback" memories of the original situations that programmed the need to tense the muscle. There is a sudden release of stored emotion, which in carthartisis clears the need for the tension to be retained. As a result the body returns to its natural posture.
Rolfers often proscribe a certain number of sessions to "unlock" the whole body at once, with a recommended sequence, usually beginning with the muscles that control breathing and the diaphram. Some people believe Rolfing to be painful, others welcome the experience because they enjoy the result.
Currently the Guild for Structural Integration, The Rolf Institute and a number of other schools teach the method as presented by Dr. Rolf. Much of the modern generic bodywork modality of 'deep tissue work' can trace its lineage back to Rolfing.