A Guang Xu "coin tree" after casting. (Image from Coole).

The Fansha or "turning sand" method.
The first detailed descriptions of coin casting is from the Ming text Tian Gong Kai Wu by Song Yingxing. Both Peng (p. 554) and Tang (p. 75) gives this explanation: 

An empty frame was made of wood. Several hundred mother coins carved of tin was arranged inside the frame. A mixture of powdered clay and charcoal was filled into the frame and pressed. Ashes from fir or willow was then sprinkled on the surface. The frame was turned over and another frame was placed on top. They were then pressed together, so the upper frame would get the impression of the other side of the mother coins. The two frames were turned over, the top one was taken off and replaced with a new frame and so on, until a pile of about ten frames could be stacked. Each wooden frame had tenons which secured that the frames did not slide. Channels were made on the surface connecting the coins, where the molten metal could flow through (see image of coin tree above). Without baking the frames, the moulds were bound together with string and were now ready for the metal to be poured in. This method improved the efficiency considerably because the mother coins only had to be arranged once, and furthermore it was not necessary to bake the moulds. 

This method was called, the "turning sand" method (Tang p. 75). The explanation is from the Ming dynasty, but it was used already in the Tang (A Xiang p. 66). Mother coins could also be made of bronze, lead, wood and stone. 

The explanation from Tian Gong Kai Wu does not tell us why the surface of the pressed clay was sprinkled with ashes, but it has surely been an agent which secured that the mother coins would slip the upper part of the mould. It was only the gravity that held the coins to the lower frame, and if the pressed clay and charcoal was the least bit sticky, the mother coins would not slip the upper frame. If the mould was sprinkled with ashes, the coins could slip more easily, and the gravity would then be enough to let the mother coins slip.