edit:Yiwu DingQiang Import and Export Co,.Ltd Update: August 17 2019.
The locking members are all alike, and therefore interchangeable, and in general form consist of contractible jaw portions which are clamped upon the tape and projecting locking portions of elongated cup shape, so that the outside of one member nests within the recess of an adjoining member when in locked relation. Consequently, it will be seen that the members on one stringer alternate with those on the other, so that when the sliding operating device is moved back and forth, the locking members will be engaged and disengaged according to the direction of movement. A further feature of the invention resides in the shape and configuration of the locking members ... [they are] provided with exterior and interior rounded surfaces, and are somewhat elongated transversely. Thereby, a snug fit is obtained and at the same time ample provision is given for movement of one on the other without coming out when the fastener is flexed transversely. At the same time this construction gives facility for relative longitudinal movement, without disengagement.
The zipper is analogous in function to a drawstring, but different in mechanism. A draw string works by tension in the string drawing together the eyelets of the piece together because the tension acts to straighten the string and so forces the eyelets toward a line. The zipper works by an elastic, that is, reversible, deformation of the "locking members" (teeth). The zipper teeth are shaped and sized so that the forces which act on the zipper when the garment it is sewn on is worn cannot unlock the teeth. The slider constrains the teeth positions, moves them along a given path, and acts on the teeth one-by-one in its "Y-shaped channel" and so can reversibly lock and unlock them. This is a lock and key design. In Sundback's invention the teeth are symmetric with "exterior and interior rounded surfaces" that are "elongated transversely". The teeth have a material part ("external projection") and a space ("internal recess"). The material part of one tooth is slightly smaller than the space on the other and so shaped to act as a "contractible jaw"--the jaw is elastically opened and then closed as it goes over the other tooth. The "snug fit" that results when "one member nests within the recess of an adjoining member" is a stable locked state. The maximum force when the slider operates is in between the unlocked and locked positions, giving two stable mechanical equilibria. The "snug fit" is stable not only to forces from wear that act in the same direction as those of the slider but to transverse and longitudinal (both perpendicular) forces.
The zipper is analogous in mechanism to a bobby pin, where the person's hand slides hair into and out of the pin's "contractible jaw".