My understing is that a chakra is an energy vortex that begins in a broarder field at the edge of the aura, it vortexes in and gets 'narrower' as you get closer to the central psychic chanells that run near the backbone like a pathway through the aura from the outside to the centre.
being a pathway and a flow of energy, a crystal, placed in that pathway will help 'align' the vibrations passing along this pathway.
What is happening on the back side is an interesting question! (thanks tarotbear) is it the same as the front, or a reverse or polarity of the energy?
I know a Tibetan singing bowl works just as well front or back. I am tending towards thinking it's the same.
Australian Aboriginals place crystals on and within the body. The ancient Egyptians placed them on the body, all over it on many energy centres, each stone carved in a specific shape (knot, ank, pillar etc)
I wrote an article once on the differing position and number of chakras in various systems (one Budhist system had 5) some Vedic systems have 6, 7 or 8 chakras. What positions are being used here?