What is the significance of the Jnana mudra or knowledge pose of Sri Krishna's hand depicted in Bhagavad Gita paintings?
Our body postures have psychological counterparts; as the mind is, so is the body. You are lying down in a particular way; that will show a certain state of your mind. You sit in a particular way; you will find your psyche manifesting in that posture in a particular way. Suppose one constantly shakes one’s legs while sitting, that shows a scattered mental state. In all these matters, the body shows the effect of the psyche. So, from that point of view, Jnana mudra is a remarkable sign of some profound psychic expression. The very name shows that, that mudra represents Jnana, knowledge - knowledge of every type; from the ordinary or secular knowledge to the highest spiritual knowledge.
Our ancient sages discovered this wonderful mudra which can describe the entire gamut of this search for knowledge; that is something extraordinary. I used to wonder about it. Later on, some years ago, when I studied biology, neurology, and allied subjects, I found one wonderful truth and that is, no animal, not even a chimpanzee, can oppose the thumb to the forefinger, but only the human child can do this. The importance of the thumb and the capacity to oppose it to the forefinger, is the beginning of man’s march to knowledge, secular and spiritual.
Therefore, in this modern age, the ideal is Jnana, and everyone is to be on the road to knowledge. Nature has given human being the capacity to oppose the thumb to the forefinger, and thus manipulate the world around him or her, and acquire knowledge and power. This is the beginning of human evolution.
Reference: Universal Message of the Bhagavad Gita Vol. 1 by Swami Ranganathananda (p.17,18,20)