The following was pulled from Healthcare 2: Examing the Subtle Body required reading Mudras for Modern Life by Swami Saradananda

Agni Mudra helps to balance the heat within your body,  and enhances your willpower and self-confidence. In the yoga tradition, it is valued for meditation, centring your enegy while filling you with enthusiasm and vitality.

You may recognize this as the thumbs-up gesture used in the West as a sign that something is a good idea, or much liked. Its origins are said to lie in the Roman era, when crowds at gladiator battles voted with their thumbs on whether the winning gladiator should kill his opponent: thumbs-up signified continued life.

HOW TO PRACTICE

Come into a sitting position [on a chair or with legs crossed or kneeling on your heels]. Each hand does something different:
     Right hand - make a fist with thumb pointing straight up.
     Left hand - open your hand, with palm facing up.
Place your right fist on your left palm and, maintaining this position, rest both hands in your lap. Hold as you sit quietly or meditate. Repeat daily.

HOW IT WORKS

By freeing your thumbs from the other fingers, you free the fire element from the influence of the four other elements [index finger = Air, middle finger = Ether, ring finger = Earth, little finger = Water]. This strengthens the nourishing fiery energy yogis call samana vayu. Holding Agni Mudra can increase your sensations of vitality and energy.

ACCOMPANYING AFFIRMATION

The positive effects of the mudra become stronger when you mentally repeat the following affirmation while holding it: 'I offer my negative qualities to the fire of transformation.'