This interaction demonstrates the water holding capacity of different soils. When a bucket of water is poured onto clay soil the strong capillary forces created by the very small pore spaces retain a large amount of the water. Only a small amount drains through to the sub soil.
When the same volume of water is poured onto sandy soil a large amount of water drains through to the sub soil. This is because the large pore spaces result in much weaker capillary forces which are unable to hold the water up against gravitational forces.