Mudras are subtle physical gesture in the body, be that with just the hands, the whole body, or a combination of asana, pranayama, bandhas and visualization techniques. The concept of a mudra is a physical movement which elicits a change or alteration in the physical body (i.e. alters our mood, attitude, perception) to deepen our connection with the subtle aspects of the body.
Looking at mudras from a posture level, all postures could be considered a mudra. That we create a gesture in the body to elicit a connection with the pranic force, thus birthing a deeper level of understanding of the effects of the asana, particularly with the subtle layers of the body, known as the ‘koshas’. The allows the practitioner to establish pranic balance and experience higher states of consciousness.
“Mudras manipulate prana in much the same way that energy in the form of light or sound waves is deiverted by a mirror or a cliff face. The nadis and chakras constantly radiate pranan which normally escapes from the body and dissipates into the external world. By creating barriers within the body through the practice of mudra, the energy is redirected within.”, Asana Pranayama Mudra Bandha, p.422.