The most common mudras for meditation is Chin mudra (palms upward) and Jnana mudra (palms down). The fingers may be joined in one of two ways:
1) Touch the tip of your thumb to the tip of the index finger to form a circle, indicating that the individual ego is joined with universal Self. Using this mudra for meditation invites calmness and helps you to tune in to a new awareness and knowledge.
2) Bring the tip of your index finger to the first joint of the thumb, indicating that the individual ego bows to the Supreme Consciousness, acknowledging unsurpassed power.
The effect of both Chin and Jnana mudras your consciousness is very subtle. But, with practice, your mind becomes conditioned. When you bring your hand into the mudra, it acts as a signal to for you to enter a meditative state.
These mudras have been found to increase the flow of blood to the brain. Thus they increase brain power and memory retention. They generate feelings of harmony, peace and communion with Universe, similar to the experience you have when you are walking in Nature. They also counteract insomnia.