The theory behind threading is similar to that of using rubber bands. Thread tied around a section of dread can, supposedly, encourage a stubborn spot (most commonly at the roots) lock up. However, unlike rubber bands which are left relatively loose, when worn at the roots, and removed after 4-6 weeks the thread used in threading is often tied extremely tight and the person to whom it is done is not instructed to remove it. They are led to believe that it can simply be left in. Usually natural fiber such as cotton is used.
Cotton, being an organic material, WILL rot over time and can cause mold growth. However, when a string is tied tightly around the dreadlock, it can cause weak spots in the dreadlock from holding the lock too tightly. Additionally, the string often “chokes” the dreadlocks causing the dreadlock above the string the hold excess moisture where it cannot as easily wick down the lock and out the end to dry properly after washing/getting wet. (See photo below for example.) For these reasons, we do NOT recommend threading and far prefer rubber bands in a situation where a client wants something placed at their roots to encourage locking.