Well beautiful question again. I try to answer.
Kashyap is a word that means turtle and it symbolizes the seal of protection or safety against danger which in our context is negative energy.
The figure of the mudra in question resembles the turtle and hence named as kashyap mudra.
This mudra can be used in countering the effects of conflicting or negative situation or new places of apprehension.
I discussed that fingers also relate to panch prana in my last answer to your question on apana-samana-mudra. As well, the fingers also correspond to chakras.
Back to basics, in our tradition in India, the thumb also relates to fire, the index finger to air (matter in gaseous form), the middle finger to ether (space), the ring finger to earth (solid matter), and the little finger to water (liquids) and the palm of the hand relates to the mind.
And therefore, most mudras are formed by using thumb (fire element) to lightly stimulate one or more of the other elements as you touch the tip, nail, knuckle or base of a finger. So, we can stabilize, decrease or increase the influence of the linked element by joining the tip of corresponding finger, touching the back of the finger (nail or knuckle) or bring thumb to the base of the finger respectively.
In kashyap mudra, by pressing down the thumb, we bring down the fire element associated with the thumb under control to control the reactive influence of negative energies. As we curl remaining fingers into the palm, all these element corresponding to the fingers become concentrated and makes your mind (subtle element associated with the palm) one pointed and at the same time, closing the palms protect this important asset i.e. your mind, symbolic of palm.
Keeping in view the above justification, i think that kashyap mudra (thumb between middle finger and ring finger) is the most stable position of hand to protect from negative energies of the circumstances, people or time. I hypothesize (may be true or not) that the change of the position of thumb either in between little finger and ring finger or in between index and middle finger may cause stability or protective effect lesser than that of kashyap mudra. If you find any other view, please feel free to share here.
