Transmission Additives-some work-But why/how does it work.

I can remember a time when I had an old Oldsmobile Delta 88 with 162k on it that developed some shift issues.  What I thought was amazing is that with a tranny flush/fill/gasket and a bottle of transmedic my tranny was fine until I sold it with 212k on it.  Another time I had a Ford Taurus whcich had a tranny that would shutter and slip.  Again I did the same thing and used a similar product maybe Lucas or something and it improved the shifting.  I would never suggest using this stuff in newer cars NOR with cars of any REAL value.  However, I am amazed on HOW and WHY they work.  Does anyone know why this stuff works?