Mudra can be translated as gesture or a seal or stamp. It is a way of holding and channeling prāṇa. We can see another kind of alchemical metaphor in it as the container that allows the transformative process to happen.
Many people think of hand gestures when they think of mudras, but this is only one kind of mudra. Hand mudras are called hasta mudras, the word hasta meaning hand. Just like how we can gesture with our hands in conversation, but can also gesture with other parts of our body or with our entire body, so other types of mudras exist. In fact, none of the mudras of the Pradīpikā are hasta mudras.
The other main types of mudras are kāya mudras, or full body gestures, and mano mudras, or mental gestures. Generally speaking, kāya mudras are the purview of Haṭha Yoga and its more physical orientation, while mano mudras are practiced within Rāja Yoga. The 3rd Chapter of the Haṭha Yoga Pradīpikā deals with the kāya mudras and the 4th Chapter, which discusses Rāja Yoga, deals with the mano mudras. Khecarī mudra, already spoken of as being the most important, has both physical and mental methods of practice. So, it is mentioned in both chapters.
The number 10 seems to have symbolic significance for the mudras as lists of mudras are almost always in groups of 10. Just as with the 6 cleansing acts or the 8 prāṇāyāmas, the number can seem a bit forced since there is some overlap between different mudras and one of them has three very different variations with different names.
The 10 Haṭha Yoga Mudras are:
1. Mahāmudra – The Great Mudra
2. Mahābandha – The Great Lock
3. Mahāvedha – The Great Piercer
4. Khecarī Mudra – The Sky roaming Mudra
5. Uḍḍiyāna Bandha – The Abdominal Lock or The Upward Flying Lock
6. Mūla Bandha – The Pelvic Lock or the Root Lock
7. Jālandhara Bandha – The Throat Lock or the Water Holding Lock
8. Viparīta Karaṇī Mudra – The Reversing Action Mudra
9. Vajrolī Mudra – The Diamond Mudra
10. Śakti Cālana Mudra – The Śakti Churning Mudra
These Mudras are said to have been learned from Lord Śiva Himself and prevent aging and death as well as granting all of the eight classic yogic siddhis, powers or attainments. Because they contain layers of subtlety and are incredibly valuable when unlocked, they should be kept secret just like jewels. One should not speak about them in the same way that one does not speak openly about the experiences with one’s intimate romantic partners.
Before leading into the discussion of each mudra, it might be nice to compare this list with a different list that is found within Śrī Vidyā Tantra. Within Tantra, mudras are rarely physical practices and instead refer to subtle energetic shifts. They do have corresponding hand gestures (hasta mudras) that go along with them, but it is clear from their names that they have deeper meanings that relate to alchemical symbolism. These 10 mudras are:
1. Saṅkṣoba – Agitation
2. Vidrava – Liquefaction
3. Ākarṣa – Attraction/Extraction
4. Vaśaṃkara – Subjugation
5. Unmāda – Intoxication
6. Aṅkuśa – Repulsion
7. Khecara – Skyroaming
8. Bīja – Seed
9. Yoni – Womb
10. Trikhaṇḍa – Three Containers/Portions
Together, they tell the esoteric story of the stages of practice within a metaphor of cooking or distilling. The agitation is the cooking and stirring of the materials, which then causes them to soften and liquefy. From this, we can extract out the essence and capture it. This essence, just like distilled spirits, is intoxicating. This divine intoxication repels us from the mundane state of things and launches us into the “sky” or inner realms. Within this we become the seed of new creation inside the womb. The three “containers” refer to the divisions of the “Knower,” the “Knowledge,” and the “Knowing” or subject, object, and verb, the divisions which allow for phenomenal existence. Note that in this list Khecarī (Khecara here) is still included, further cementing its preeminent importance.
Even when it comes to the more physical kāya mudras of Haṭha Yoga, there are still gross and subtle levels to their practice. The outward physical posture or activity is just a way of signaling some kind of energetic shift. If this shift is understood on its own terms, then sometimes it can be effected without taking the outer form of the mudra. This is how one progresses towards Rāja Yoga. The practitioner is thus encouraged to try to “look under” the mudra and see the essence behind what is merely apparent about it.
 Vajrolī mudra, which includes Sahajolī and Amarolī mudras as well.