The front side of the body is called east side, while the back side is called the west side. In this Asana complete back side, i.e., the west side, right from the heels to the forehead, gets stretched and therefore, it is called 'Pashchimottanasana'.
As the whole of the west side gets stretched, they get purified and their functioning is greatly improved. At the same time, the muscles of the front side get contracted and there is pressure on the lungs, intestines and other internal juice producing glands. As a result of this their functioning is also improved.
It is very useful in setting right the troubles concerning the lumber part of the spinal cord and the digestive system.
Further,there is very sensitive part in our body known as Kundalini lying in dormant stage. This Asana is supposed to activate it.