The zipper’s locking function is automatically activated when there is no load on it, according to YKK’s official website. Means: If the small part with which you open the zipper is pointing downwards, it is firmly closed and cannot open. Now straighten the small handle a bit so that it protrudes, the zip fastener is unlocked and it opens easily.
More precisely, you activate this function every time you want to open or close a zipper, because to do so you grasp the handle between your fingers and thus unlock the mechanism.