The Dai-Ko-Myo is known as the Master symbol. It activates the other Reiki symbols in a more focused way. This symbol is very powerful because it not only focuses the Reiki energy onto the spiritual level,  but also connects to the Soul or Higher Self.

The Dai-Ko-Myo works on the heart Chakra and opens it up to healing, cleansing it to allow for better communication and healing from the higher vibrational levels of the energy system.  

This symbol clears the communication channels with the realm of the Guides, i.e., it helps to lift the veil between the two worlds slightly so that communication is possible. It works on not only the heart Chakra but on the throat the thymus as well as the crown Chakras