Vacuum the subfloor around the doorjamb.
Place a piece of scrap material of the flooring --- tile or hardwood --- on the ground next to the doorjamb.
Place the jamb saw on the subfloor next to the scrap flooring and adjust the saw blade so the top of the blade is even with the top of the flooring scrap.
Adjust the depth of the cut to approximately 7/8 inch.
Place the jamb saw on the floor next to the casing and start it. Drive the saw blade into the casing on one side of the doorjamb, and rotate the saw around the doorjamb, cutting the bottom of the jamb with the saw.
Repeat this process on the other side of the door.