At the end of the Ashtanga Yoga practice every Ashtangi does the same three postures: (1) yoga mudra - 10 breaths; (2) padmasana with a hasta (hand) gesture (mudra) known as jnana (gyan) mudra - 10 breaths; and the dreaded (3) utplitih. There is great meaning and symbolism packed into this 30 breath tradition. One place of special significance is the hand gesture - jnana mudra - that we take in the second of the three. 

One takes jnana mudra by gently pressing the thumb and forefinger together to form a circle and then extending the other three fingers out and pulling them together. Then we extend the arms and rest the backs of the wrists on the top edges of the corresponding knee, sit up tall, slightly tuck the chin and shift the soft gaze to the tip of the nose (nasagrai drishti). There we breathe deeply in and out ten times using slow, strong breath-with-sound.

Each component of this gesture carries deep meaning. Overall it is said to channel pranic energy to revitalize our being and overcome the negative impacts of ego. We usually release prana through our extremities; however, by forming this loop we retain some and send it inward to help calm and steady the mind to provide support for the meditative state.

The thumb represents the Supreme Reality called Brahman. Depending on your personal belief system this may be translated as the absolute truth, cosmic consciousness, God, Isvara, Purusha, the great void, and so on. The forefinger represents the individual soul or the Jiva or our inner reality. Here this could mean your innate wisdom and compassion, Buddha Nature, spirit, essence, purusha, or truth.The important thing here is that you assign meaning that feeds your spiritual path and awareness.

The two fingers touching each other signify the oneness of Brahman and Jiva. In other words the Individual self is not different from the Supreme reality Brahman. It’s the wisdom of knowing they are one - the “union” of yoga - overcoming the samyoga or confused state we’re usually in wherein we fail to distinguish the material from the spiritual (avidya).

Also, the three other fingers represent the three Gunas of Nature. The middle finger represents sattva guna (light, purity, clarity of mind, understanding, etc.). The ring finger represents rajas (action, passion, movement, etc.). The little finger represents tamas (inertia, dullness, inaction, confusion, etc.). These are the three primordial forces of nature that are operating within and around us at all times. These three Gunas have to be transcended to reach the ultimate reality. That is the philosophical symbolism behind this gesture.

tat paraṃ puruṣakhyāter guṇavaitṛṣṇyam || 1.16 ||

Higher than that, from discerning the Puruṣa, [arises] the lack of desire for the Guṇas.

(Seth Powell, Yogic Studies)

 One of my favorite inspirations for this posture, mudra and its practice comes from an old video of Pattabhi Jois teaching it to Richard Freeman. You can find that video here: https://www.youtube.com/watch?v=JUh5JHmck4k and this is a snapshot I grabbed of the mudra.