This is hardly a knot at all, relying essentially on friction rather than on binding. It is mostly used when relieving the load on one line by attaching another, such as when trying to undo a riding turn on a winch or sharing the load on a mooring cable.
A turn is taken with the relieving line’s working end so that it crosses over itself (as in the photo below left). The end is then rolled on around the loaded cable for several turns before either being held by hand or half hitched to the cable. Load is then applied to the relieving line. The Stopper Knot can be undone even under load by releasing the working end and casting off the turns.