Purna Gyan Mudra
This is one of the advanced version of Gyan Mudra and is said to be the provider of wisdom. It is said that Lord Gautam Buddha was in this mudra when he received enlightenmentand also, in ancient indian religious texts , it is said that Lord Shri Krishna was in this mudra when he gave the delivered the knowledge of Gita to Arjuna.I think these examples are good enough to enlighten you to to importance and significance of this mudra.
How to form Purna Gyam Mudra:- Sit in Padmasana or Siddhasana which ever is comfortable to you. Now bring your hands in Gyan Mudra and keep one hand near knee and right hand near chest region and sit as long as you meditate or as long as possible.
Benefits:- It introduces the sense of spiritual stamina and dispassion,improves memory to a great extent,bestows intelligence and wisdom,causes electrical changes in the electromagnetic activity of the brain leading to increased clarity and understanding.It also soothes irritable tempraments.
Precautions:- This mudra should not be performed if you are in high-strung. In fact, meditation is not recommended in such states. Instead, do some pranayams and then meditate in purna gyan mudra.