Walt Fritz has written here about some of the issues involved. I will try and summarise them.
Massage often works in reliving pain, stress etc. However the mechanism by which this happens is often poorly understood, even by therapists. Walt states that he is much less specific about how what he tells clients.
Examples of what clients believe massage does that have been disproved by research are that it gets rid of toxins in the body, and that it gets rid of lactic acid in the muscles after exercise.
This is not to say that massage is any less effective, just that our understanding of how it works is less than perfect.
This does leave me with a dilemma when a client tells me that one of the main reasons for coming is to get rid of toxins from their body. I usually try and sound them out a bit about this belief they have. If it is a very strong belief I might just suggest that diet, avoiding alcohol etc. might make more difference. If it is a vague idea I might point out that the latest research suggests that this doesn’t actually happen. What I don’t want to do is get in an argument with them over it which will lose me their custom!
What the latest research does suggest is that the effects of massage are much more through it’s effect on the nervous system than had been realised in the past rather than releasing knots and other non-existent (at least in a literal sense) conditions.