On the Standard ( or Gallows ) there's a peg, on the strap holding the bag below is a hole, as the strap is placed on the peg a shaped piece of metal acts as a catch.
On the Traductor arm, there is a similar arrangement but this has a snap down catch to hold it in place and just to make sure that the catch doesn't come open, it's tied with 'hairy ' string, this gives the right type of break when the pouches are being dropped off. That's me, hanging up the pouches, towards the end of the video.