If switch is set to -yes then each -quad4 element is split into four -tria3 elements and each -hex8 element is split into twelve -tet4 elements. Further, each -quad9 element is split into four -tria6 elements and each -hex27 element is split into six -tet10 elements.
See the rules for old and new at control_mesh_refine_globally. See also control_mesh_split_only.