Shambhavi Mudra is a body posture in which your concentration power increased many times.
When you take Shambhavi mudra in meditation a energy circuit has been completed and you become more focused.
If you want to do practical I would suggest it also,PL DO IT.
In the meditation leave your hands freely whatever you want and next time do meditation with Shambhavi mudra you will get the benefit of this and you will understand more better than believe.
Most important thing in shambhavi mudra is just touch your index finger tip with your thumb tip of both hands gently do not soo tightly and do not soo loose and you may see it in image also.