The triskell is held as a symbol of balance between heaven, earth and sea, and between the worlds of men, spirits and gods. It´s the key to moving between the levels of existence.
Similarily the yin and yang indicates balance, and it´s the key to harmony and perfection, the balance between male and female, light and darkness, good and bad... As light carries shadows, darkness makes light seem brighter.
This version was thought for the lower back, or for the higher back with the triskell symbol placed at the base of the neck.