Both sides of each sliding section has locking pushpins.  These locking pins anchor down into the slide rail holes securing each sliding section to the rails. In order to slide the section, the pins must first be detached from rails by pulling up and out of anchor holes in the slide rails. If one forgets to anchor down all six push pins, the SLIDERTOP could slide out of the back of rails.  To prevent this from ever happening, each rail has removable back up stops.  See pictures below for reference.