This mudra is like the snakes hood. When the Hand is held in Pataka Mudra and the fingers are bend from the tip to form a hollowed palm we get Sarpashirsha Mudra.

This Mudra is used to denote the following.

A Snakes Hood
Sandal Paste
Middle tone
Sprinkling
Nourishing
Giving water to Gods and sages
Moving to and fro
Movement of protrusion of Elephants [...]