That which grants Muda or bliss is Mudrā:
mudaṁ dadātīti, mudaṁ rātīti mudrā ||
That which liquefies or dissolves pāśa or bondage is mudrā:
muṁ drāvayatīti mudrā ||
That which locks up or seals is mudrā.
mudrayatīti mudrā ||
Abhinavaguptapādācarya quotes the following verse in his Tantrālōka:
mudaṁ svarūpalābhākhyaṁ dēhadvārēṇa cātmanā |
rātyarpayati yattēna mudrā śāstrēṣu varṇitā ||
That, which grants the bliss of svasvarūpa jñāna encompassing the three states of jāgrat, svapna, and suṣupti, is called mudrā.
From these definitions, the goal of using mudrā becomes evident.
Mudrās can be hasta-mudrās used in āgama and Tantra or the yogic (Haṭha yoga to be specific) mudrās, though the two are intrinsically related.
The use of Tāntric mudrās is twofold generally, as described in Svacchandabhairava Tantra. It affects the microcosm and the macrocosm. The evident use is in specific rituals. For example, Garuḍa mudrā is used for nirviṣīkaraṇa or removing the subtle poisons present in the dravya to be offered to Yōginīs. Yōni mudrā removes the dōṣas in the mantra and in Japa. Kañkāla mudrā arrests the offensive thoughts of the opponent. Trikhaṇḍā is used for āvāhana of Tripurā and so on.
Also, each of the mudrās has specific energetic effects on the sūkṣma śarīra. For example, Yōni mudrā awakens and unifies prāṇa in the three meridians. Tārkṣya mudrā affects the flow of vyāna and hence affects circulatory and digestive systems and so on. When one’s nāḍis are appropriately cleansed and one is sensitive to the flow of prāṇa within the body, the subtle effects of these mudrās can be easily experienced and identified. For example, when holding the Trikhaṇḍā mudrā, one can feel the unity of Mūlādhāra, Anāhata, and ājñā, and the primal prāṇa gushing upwards like a burst of a warm jet. When Kūrma mudrā is held, one can feel the shielding or insulating of Maṇipūraka by the sensations of prāṇa in that center.
Mudrās have been adopted by various mystical cultures and examining them will show the validity of the different definitions stated above. The Samurai schools have, at the simplest level, Kuji-in and Kuji-Kiri, which are basically energy seals. These seals, generally 81 in number, work at sealing the various forces of the body and activating a specific meridian or a channel. The focus of many here is the heart center. The Daoist mudrās are chiefly to manipulate the five elements (earth, water, fire, metal, and void) in the body to achieve specific goals and these are focused mainly at the three cauldrons or the Dantiens, especially the crucial lower Dantien. There are many mudrās here that not only seal but re-direct or stream energy outwards, somewhat like a beam of laser (Fajin). These are especially made use of in martial arts. The Tantric mudrās are much more expansive in their field of activity and address a wider arena of goals. The Tibetan mudrās of Yantra Yoga or Trul Khor are comparable to the classic Haṭha yōga mudrās but incorporate in them, the subtle aspects of manipulating sūkṣma śarīra as in the case of Tāntric mudrās.
Some practical uses of mudrās have been illustrated by enlightened masters. For example, an acquaintance recollected her father’s experience with Sri Abhinava Vidyā Tīrtha, a great Yogi who had achieved the Khēcarī Mudrā. Her father was suffering from severe arthritis and the ācārya saw him limping. He called him close to him and enquired about his spiritual practice. He then instructed him to practice a certain prāṇāyāma while holding the Kūrma mudrā, between 5 to 7 am in the morning and stop drinking milk. Needless to say, pain reduced considerably within a short period of practice. When one examines the combination of this prāṇāyāma and the effects of Kūrma mudrā, we can easily see the logic behind ācārya’s instruction. So, yes, there are such day-to-day uses of mudrās as well, but require a deep understanding of the involved mechanics before they can be put to use.