[Query] Namaste, I am highly interested to know the practical use of Mudras?
That which grants mudrA or bliss is mudrA [mudaM dadAtIti or mudaM rAtIti].
muM drAvayatIti mudrA, that which liquefies or dissolves pAsha or bondage is mudrA.
mudrayatIti mudrA, that which locks up or seals is mudrA.
Abhinavaguptapadacharya quotes the following verse in tantrAloka:
mudaM svarUpalAbhAkhyaM dehadvAreNa chAtmanA |
rAtyarpayati yattena mudrA shAstreShu varNitA ||
That, which grants the bliss of svasvarUpa j~nAna encompassing the three states of jAgrat, svapna and suShupti, is called mudrA.
From the definition, the goal of using mudrA becomes evident. I assume it is mostly the third aspect of mudrA that your question is related to, though they are all interconnected at some level.
mudrAs can be tantric hasta mudras with specific purposes or the yogic [haTha yoga to be specific] mudrAs.
The use of tAntric mudrAs is twofold generally, as described in svacChadabhairava tantra. It affects the microcosm and the macrocosm. The evident use is in specific rituals. For example, garuDa mudrA is used for nitviShIkaraNa or removing the subtle poisons present in the dravya to be offered to yoginis. Yoni mudrA removes the doShas in the mantra and in japa. Ka~nkAla mudrA arrests the thoughts of the opponent. trikhaNDA is used for AvAhanA of tripurA and so on. Also, each of the mudrAs has specific energetic effects on the sUkShma sharIra. For example, yoni mudrA awakens and unifies prANa in three centers or meridians. tArkShya mudrA affects the flow of vyAna and hence affects circulatory and digestive systems and so on. When ones nADis are appropriately cleansed and one is sensitive to the flow of prANa within the body, the subtle effects of these mudrAs can be easily identified. For example, when one holds trikhaNDA mudrA, one can feel the unity of mUlAdhAra, anAhata and Aj~nA, and the primal prANa gushing upward like a burst of warm jet. When kUrma mudrA is held, one can feel the shielding or insulating of maNipUraka by the reduced activity of prANa in that center.
Mudras have been adopted by various cultures and examining that will show the validity of the different definitions stated above. The Ninjas 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, the left heart, unlike the valuable right heart that bhagavAn ramaNa talks about. The daoist mudrAs 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 tan tiens, especially the lower tan tien. There are many mudrAs here that not only seal but re-direct or stream energy outwards, somewhat like a beam of laser. These are especially made use of in martial arts. The Tantric mudrAs are much more expansive in their field of activity and address a wider arena of goals. The Tibetan mudrAs of Yantra Yoga or Trul Khor are comparable to the classic haTha yoga mudrAs but incorporate in them, the subtle aspects of manipulating sUkShma sharIra as in the case of tAntric mudrAs.
These days, people learn a set of mudrAs and then try to figure out their use and to fit them in their practice. This is an extremely incorrect approach to learning mudrAs. One should rather learn the chief practice and then adopt mudrAs that are required by the practice which ensures their compatibility with the practice. Some rather practical uses have been illustrated by enlightened masters. For example, an acquaintance recollected her father's experience with H H Sri Abhinava Vidya Tirtha Mahaswamigal, a great Yogi and a realized master. Her father was suffering with severe arthritis and the AchArya saw him limping. He called him close to him, and enquired about his spiritual practice. He then suggested him to practice a certain prANAyAma while holding the kUrma mudrA between 5 to 7 in the morning and stop drinking milk. Needless to say, pain reduced considerably within a short period of practice. When one examines the prANAyAma and the effects of kUrma mudrA, one can easily see the logic behind AchArya's instruction. So, yes, there are such day-to-day uses of mudrAs as well, but require a deep understanding before they can be put to use.