Yoga Hand

What is the importance of "Hast(hand) Mudra" science? Why lord Krishna preach Geeta to Arjuna in Gyaan Mudra?

Also state their importance in Yoga.

In Hindi:-
"हस्त मुद्रा" विज्ञान का क्या महत्व है? श्रीकृष्ण ने अर्जुन को ज्ञान मुद्रा में ही गीता का उपदेश क्यों दिया?
योग में इसका क्या महत्व है?

Sri Vidyaji is more competent to reply on the second part of your question but I would like to share some information on the first part of your question.

Fingers have always had a important part in both Yoga and Vedic duties performed during sandhya, pooja, marriage, etc. Today these vedic duties tend to be more ritualistic and the importance has lost it meaning.

For some time I have been compiling information on health benfit of various Mudras and would like to share with you the same. While I practice them myself most of them to whom I have recommended have acknowledged its benefits.

1. Gyan Mudra (Mudra of Knowledge):
Method:
Touch the tip of the thumb to the tip of the index finger, with the other three fingers stretched out.
Specialty:
As it is a mudra of knowledge, it enhances the knowledge. The tip of thumb has centers of pituitary and endocrine glands. When we press these centers by index finger the two glands work actively.
Time duration:
There is no particular time duration for this mudra. You can practice by sitting, standing or lying on bed whenever and wherever you have time.
Benefits:
-Increases memory power and sharpens the brain
-Enhances concentration and prevents Insomnia
-If we practice it regularly, it will cure all psychological disorders like Mental, Hysteria, Anger and Depression

2. Prithvi Mudra (Mudra of Earth):
Method:
Tip of the ring finger touches the tip of the thumb, with the other three fingers stretched out.
Specialty:
It reduces all physical weaknesses.
Time Duration:
It has no particular time duration. You can practice it any time you want.
Benefits:
- It helps to increase the weight for weak people
- It improves the complexion of skin and makes the skin to glow
- It makes the body active by keeping it healthy.

3. Varuna Mudra (Mudra of Water):
Method:
Tip of little finger touches the tip of thumb, with the other three fingers stretched out.
Specialty:
It balances the water content and prevents all diseases which come due to lack of water.
Time Duration:
It has no specific time duration and one can practice it according to their time.
Benefits:
- It retains clarity in blood by balancing water content in the body
- Prevents the pains of Gastroenteritis and Muscle Shrinkage

4. Vayu Mudra (Mudra of Air):
Method:
Keep the index finger on the base of the thumb and press with thumb keeping the other three fingers straight.
Specialty:
It prevents all the diseases that occur due to the imbalance of the air.
Time Duration:
The practice of this mudra for 45 minutes reduces the severity of the disease in 12 to 24 hours. For better results practice it for two months.
Benefits:
- It cures Rheumatism, Arthritis, Gout, Parkinson's disease and paralysis without any medicine
- It is useful for Cervical Spondilytis, paralysis to face and catching of nerve in neck
- It corrects the disorder of gas in the stomach

5. Shunya Mudra (Mudra of Emptiness):
Method:
Keep the middle finger at the mount of Venus and press it with thumb.
Specialty:
It reduces the dullness in our body.
Time Duration:
One can practice it for 40 to 60 minutes daily until to be cured from the disease.
Benefits:
- It relieves an earache within 4 or 5 minutes
- It is useful for the deaf and mentally challenged, but not for inborn ones.

6. Surya Mudra (Mudra of Sun):
Method:
Bend ring finger and little finger and touch the tip of thumb with their tips keeping the remaining two fingers stretched.
Specialty:
As it is the mudra of life, it improves the power of life. Weak people become strong. It reduces the clamps in blood vessels. If we practice it regularly, we will become active.
Time Duration:
No specific time duration. One can practice it any time.
Benefits:
- It improves immunity
- Improves the power of eyes and reduces eye related diseases
- It removes the vitamin deficiency and fatigue

8. Apana Mudra (Mudra of Digestion):
Method:
The tips of middle finger and ring finger touch the tip of thumb while the other two fingers are stretched out.
Specialty:
It plays an important role in our health as it regulates the excretory system.
Time Duration:
Practice it daily for 45 minutes, but practice for longer time yields more benefits.
Benefits:
- It regulates diabetes
- It cures constipation and piles
- It helps excreting the normal waste regularly

9. Apana Vayu Mudra (Mudra of Heart):
Method:
The tips of the middle finger and ring finger touch the tip of thumb, while the index finger touches the base of thumb and little finger stretched out.
Specialty:
It benefits the heart. It works like injection in the reduction of heart attack. It is as powerful as sorbitate tablet. It reduces the gas content in body.
Time Duration:
Practice it as many times as you can. Heart patients and BP patients can practice it for 15 minutes daily twice for