Standing in the center of Reiki practice is the ability of the practitioner to bring down energy from the Heaven (masculine) and bring up grounding energy from the Earth (feminine).
These two energies unite with the individual's potential inner heart energy. Bringing balancing to the person. Relaxing and energizing.
People are indeed happier when they live through their heart. Healing comes from the heart.
Reiki is the 7th and ultimate energy on the physical plane which brings balance to all other energies in a persons life.
The other six energies are:
These energies do not directly or necessarily correlate with the seven chakras...but I say do the research and meditate on this for yourself and come to your own conclusion.
You can learn more about the seven energies represented in Japanese tradition by reading THE SPIRIT OF REIKI by Lubeck, Petter and Rand.