The background is that most of us have had lives which have contained some more or less unpleasant or stressful incidents. One tenses up temporarily to get through the immediate situation. The only problem is that your muscles get used to the tension, so that after a time, you do not feel it any more. Locked-in, repressed tensions can then lead to all sorts of problems later on in your life.
In a session, the practitioner then gently searches for the client's tensions, and when he/she finds one, just holds against the the muscle tension. After a while, the client will begin to relax and could well experience something of the situation which caused the tension in the first place.
The practitioner encourages the client to talk, and unload something of the background. The emotional release is subtle and powerful.
This may sound a bit technical - Ulrika Tham explained it a lot better than I could.
For more information, you could contact the Rosen Institute in Berkeley.