The “forced notch” mechanism is an evolution of the traditional slipjoint system, and often found on French knives such as Laguioles, Saint-Martin or Thiers for instance.
It’s somewhere between a classic flat slipjoint and a lockback.
The blade has a little notch where a part of the backspring comes to lock. Unlike a lockback, it’s rounded and will slip when pushing on the blade to fold the knife. It’s an added safety to prevent the knife from closing involuntarily.
The lock must be “broken” before the blade can be folded slowly in the handle.
