According to the patent’s description, “The watch band maintains a substantially constant tension throughout changes in its length while worn by a user. Such changes in length may occur automatically to accommodate changes in the size and circumference of a user’s wrist as they move their wrist normally. By maintaining a constant tension, the watch band also maintains a constant force on the user’s wrist, and causes a watch body attached to the watch band to also maintain a constant force on the user’s wrist.”
The description adds, “This can increase a user’s comfort, since the watch will not get tighter or constrict their wrist as they straighten and bend their wrist. It can also help optimize operation of any sensors in the watch band or watch body that benefit from being held against the user’s wrist with a constant force, such as some physiological sensors.”
It is unclear how exactly Apple will achieve this and what kind of materials they will be looking to use. However, it is not a guarantee that this patent will become a reality as Apple does file for patents regularly.