Past life regression works under the theory that the immortal soul is transferred through the ages from one living being to the next. It could have been Cleopatra, it could have been a small squirrel. Whatever the previous being, one cannot consciously know about these past lives, because the memory of these former existences is erased. However, the information is still stored inside the soul and can be retrieved. Hypnosis, of course, is the preferred method.
Hypnosis taps into the subconscious mind. Through this, you can find out all sorts of things about yourself. The subconscious mind's stores of knowledge are hidden from the conscious mind. However, the subconscious mind does quietly communicate information to the conscious mind that affects the way the conscious mind behaves. Thus, if you want to learn what's inside the subconscious mind, or change the information, you can use hypnosis to rummage through the contents.
If you want to know just what a previous lifetime is telling your current lifetime, you can delve into the subconscious mind that keeps that information as well. Past life regression is often employed to see what is in there and dig out what, say, Schubert is telling you to do.
Past life regression is a very controversial use of hypnosis, to say the least. The mysteries of the soul and its fate after the death of the body are big enough even without worrying about how to contact your previous iterations. However, if you do believe in reincarnation and if you do believe your previous lives can teach you something about your own, there are many hypnotists that can assist you in your quest.
