Thappi is a term used in Rajasthan, I do not what they call it in other parts. It is the process of gently tapping the surface when the process of hardening has started. Without thappi, the practice of lime-surkhi is incomplete. It will not have enough strength to hold itself, it will be loose and will be prone to more cracking. It is also a process of binding and ingraining the layer of the plaster to the base layer.
As soon as the coat of plaster or roof is done with surkhi, it is lightly tamped with a wooded piece. Then as it becomes dry, the intensity of tamping is gradually increasing till you hear a ringing sound. The first tapping makes sure that the coat sticks well to the wall or the substrate and sets well with it. It also prevents cracking and gives strength to surkhi. This is the most important step that cannot be ignored. I have seen that my fellow villagers always do this with lime works.