One of the current tenets of orthopedic
fixation is that bone heals better if the fracture fragments are pressed firmly together. Screws that push the broken bone fragments together are often used for that.
The Herbert screw is designed for use in fractures of small articular bones such as the carpals. It is cannulated and threaded at both ends. These threads run in the same direction, but the proximal portion has a wider pitch to its thread.
Thus, when the proximal threads engage in the bone, they tend to move through the bone faster than the threads at the distal end, causing the two ends of the bone to compress together. This screw is used where a standard screw would impinge on adjacent tissues, such as in the treatment of scaphoid or osteoarticular fractures.
The Herbert screw is not a substitute to full healing so you should not engage in physical activities until it's taken out and the doctor tells you you can. It is basically a substitute for a cast and makes your bone more likely to heal. Doctors sometimes differ on the criteria for when the Herbert screw should be used on a fresh fracture but most if not all recommend it for non-unions.
Source: Approaches To Differential Diagnosis In Musculoskeletal Imaging, Michael L. Richardson, M.D., Orthopedic Hardware and others