The mechanism consists in a couple of lids mounted into the side holes of the spool. These lids are having 4 claws each and they slide on the smooth surface of the rod. The tips of the claws are gently pushing against the rod's surface and this uses the elastic properties of the PLA plastic. For more details on how to install or print them, you can also check the thingiverse link below:
After you finish the filament, the lids can be replaced to anew spool. They easily snap into position and they can also be easily removed.