The antagonism between the muscles that flex and extend the ankle is often very strong, much stronger than the gravitational stress on the navicular bone that stands as the keystone of the arch of the foot. This pulling at the forefoot and hindfoot is usually responsible for tearing the fibres of the plantar fascia.
The pulling can be compared to an archer pulling backward at the extremes of bow, and if the forces get great enough, either the bow (foot) will break or the bowstring (fascia) will tear. Yoga works to relieve the antagonistic spastic-like stress indicated by the arrows in the picture.
Once it is resolved, the plantar fasciitis heals on its own in a matter of 2-3 weeks. Yoga, Alexander and Feldenkrais occasionally need help with an ultrasonically guided steroid injection, but not often.