Lag screw principles for cortical bone
The threads only engage in the bone on the far side of the fracture. The hole on the near side of the fracture, the 'gliding hole', is larger so that the portion of the thread in the gliding hole does not purchase in the surrounding bone.
As the screw is tightened, the thread pulls the opposite bone fragment towards the head of the screw, which compresses the near fragment against the opposite fragment.
Insert the lag screw as perpendicularly to the fracture plane as possible.