Yin must be able to cool the body adequately,
so that Yang doesn't warm the body excessively.
E.g.: The Yin of the Kidney cools the Yang
(fire) of the Heart.
To moisten:
Yin moistens the body, including the skin,
hair, muscles, mucous membranes etc.
E.g.: The Yin of the Lungs moistens the
skin.
To nourish:
Yin nourishes the body.
E.g.: The Yin (Blood) of the Liver nourishes
the muscles and the tendons.
To calm:
Yin calms the activities of Yang. The calming
function of Yin protects us from excessive activity and restlessness.
E.g.: The Yin of the Heart calms the spirit.
If Yin is weak, the functions of Yin can not be
maintained. As a result, Yang will dominate in the body.