This is another version of the chin or gyan mudra (wisdom gesture). The gyan mudra is the most common gesture used by meditators. Purna gyan is obviously more difficult to perform, since it calls for immense stamina.
Sit in a meditative pose. Place your hands in the gyan mudra: tip of index finger touching tip of thumb, for each hand. Hold your right hand in the gyan mudra level with your right shoulder. Lower your left hand towards your left knee, holding it there. Ensure your back is straight and eyes are shut. Hold for as long as is comfortable, gradually increasing time to be able to hold it for the entire duration of your meditation.
Avoid: If you are in a high-strung or high-vata (air element) state. In fact, meditating in such a state is not recommended. Ideally you must practice calming pranayama (breathing) practices like ujjayi or grounding asana like shashankasana in such states.
Benefits: Introduces the sense of spiritual stamina and dispassion.
Also read: Have fun with yogic hand balancers