This gesture consists in powerful contraction of the anal sphincters in order to influence the base (Mulàdhara). 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.
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.
Anatomical Reference