As the thread advances in the main body of the tibia, the head pushes the medial fragment towards the tibia.
The smooth shaft of the screw prevents any significant hold between the screw and the surrounding bone.
The length of the screw shaft must be chosen so that the threaded part of the screw lies fully beyond the fracture line and within the epiphyseal cancellous bone of the distal tibia.
To prevent the screw head from sinking into the thin cortex, the use of a washer is recommended.
In a vertical fracture, provided the fragment is large enough and the bone quality is good enough, two or even three lag screws on their own may be sufficient. If in doubt, a buttress plate may be more secure.