The thread pulls the head of the screw and therefore, the malleolar fragment towards the main body of the tibia.
The smooth shaft of the screw prevents any significant hold between itself and the surrounding bone.
The length of the screw chosen is based on the principle that all thread must lie proximal to the fracture plane. Be careful not to choose a screw that is too long, as its thread will come to lie in the more fatty cancellous bone of the metaphysis, rather than in the denser bone of the former epiphysis.
To prevent the screw head from sinking into the thin malleolar cortex, the use of a washer is recommended.