The technique of separating that is used by John in this experiment is called "filtration". This is a process by which the mixture is allowed to pass through a medium that has mesh. The mesh needs to have hole size that is smaller than the particle size of the substance that is desired to be separated.
In this experiment, the mesh should have hole size smaller than the particle size of the soil for it to retain in the mesh while the mesh will allow passage of the water particles.