My mind = blown:

The design has no locking mechanism, and it is just inertia of the barrel traveling backwards that holds everything together until the bullet leaves the muzzle (as with any blowback design). The recoil energy from the shot has no way to push the breech of the gun rearwards, and so instead it acts on the barrel, pushing it forward against the recoil spring until it locks in place, ready for the next shot.

When he pulled that barrel forward in the video I was like =)

What a neat design, very simple too.  I had no idea this even existed.

You can read more about it on Forgotten Weapons.

Hat tip: Anders