Chin mudra:

You have probably seen yogis holding their thumb and pointer finger together and extending their remaining 3 fingers out. This is chin mudra. Chin means consciousness in Sanskrit, and the purpose of this mudra is to remind the practitioner of the goal of yoga, the union of the individual soul with the supreme soul. Mudra means seal, and it is essentially an energetic and spiritual gesture that controls the flow of energy within the body. The fingers each have representations:

• Thumb: Supreme Soul
• Pointer: Individual Soul
• Middle: Ego
• Ring:karma
• Pinky:illusion
  
  

Ego, illusion and karma are the 3 impurities that the yogi is trying to remove from his life in order to unite their Individual Soul with the Supreme Soul and experience that divine, blissful state of union they strive for.The pointer surrenders to the supreme soul's leg. Doing chin mudra is a physical representation and reminder of this goal and serves to refocus and re-energize the practitioner. Try it and see how it makes you feel. Sit cross-legged, straighten your spine, close your eyes, bring both hands into chin mudra and rest your hands on your knees. Think pure thoughts and feel connected to the entire planet.

Scientific Meaning of chin Mudra:

 
The circle formed by the index finger and thumb represents the true
goal of yoga – the merging of the individual soul with the universal
soul, or the soul of God. The nail of the index finger is placed
into the first joint of the thumb. The last three fingers always
face down towards the earth in this mudra.

                    It is a gesture of receiving. When the finger touches the thumb a circuit is produced which allows the energy that would normally dissipate into theenvironment to travel back into the body, and up to the brain. 

                     When the fingers and hands are placed on the knees the knees are sensitised creating another pranic circuit that maintains and
redirects prana within the body. In addition placing the hands on
the knees stimulates a nadi, which runs from the knees up the inside
of the thighs and into the perinium. Sensitising this channel helps
stimulate the energies at the muladhara chakra. When the palms face
upwards in the chin mudra the chest and heart area are opened up.