A half-stop is a position roughly halfway through a slip-joint knife's travel where the blade pauses and holds. It comes from a flat ground into the blade tang that the backspring rests against, and it makes the knife safer to close by breaking the movement into two stages.
Without one, a slip joint goes from open to shut in a single snap driven by the backspring. With one, the blade stops halfway, giving you time to move your fingers clear before completing the movement.
On a knife with a strong spring, that pause prevents a genuinely painful pinch.
A crisp, well-defined half-stop is one of the things collectors judge fit and finish by, alongside walk and talk. A vague or mushy one suggests loose tolerances.
Its absence is not a fault. Many excellent patterns go straight from open to closed by design, and different makers use it on different patterns.
A half-stop holds the blade at a resting position. It does not lock anything, and a slip joint can be closed by pressure on the spine at any point in its travel.
See the glossary entry for the mechanism.