Ashvini Mudra अश्विनी मुद्रा
Ashvini: From अश्व. aśva, a horse. Also spelled ashwini. The term ashvini अश्विनी (aśvinī), the "wife of the Ashvins (two Vedic gods, divine twin horsemen", means the head of Aries or the first of the 28 nakshatras (see Ashvini and Ashvins). A slightly different spelling for it (आश्विनी āśvinī) is the name of a month in the rainy season.
Note: With regard to the symbolism of the horse, see the Ashvamedah ritual (horse sacrifice, अश्वमेध)
This gesture, also called 'dawn horse gesture', consists in powerful contraction of the anal sphincters in order to influence the base (Muladhara). One proceeds by sequences of contractions and relaxations. Most often, this mudra is done during retentions of breathing (full or empty).
Yoga techniques encompass all parts of the body in order to reach each aspect of the energies. The base is a crucial center as it allows the progressive awareness of muladhara.
Side View of the rectal area.
More generally, to reject parts of our energy systems, i.e., by trying to contain the energies linked to them, can easily linked to havoc movements such as inappropriate anger.
Through yoga, or other energy control systems like acupuncture, these energies are progressively addressed and their movement regulated.