Double-Knock mechanism

Some double-knock pencils. Left to right: three Niji Grip 350DX (the black one is a vintage model), two Niji Grip 0.9mm, a Pilot Delful The Shaker, and a Serve Swell

Double-knock or vanishing point (also dual action), is a particular type of mechanism that allow the tip of a mechanical pencil to fully retract inside the body of the pencil. In a double-knock pencil, a deep click of the push button extends the whole tip section, while further short clicks allow the lead to come out (like a regular mechanical pencil). In order to retract back the tip (when it's in the extended position), usually another deep push of the button is needed.

Double-knock pencil models

Brand	Model name	Additional Features
Pilot	Vanishing Point	Lead hardness indicator
Pilot	Automac	Auto advance mechanism; metal body
Pilot	Delful The Shaker	Shaker mechanism with lead lock when the tip is retracted
Pentel	Graph Gear 1000	Spring loaded clip to retract the tip; metal body; lead hardness indicator
Pentel	Technica-X
Pentel	Flex Fit	Spring loaded clip to retract the tip; ergonomic rubber grip (also customisable in the Flex Fit II)
Rotring	Rapid	Twist-extendable eraser
Niji	Grip 350DX	Twist-extendable eraser
Ohto	Horizon	Auto advance mechanism; side button to retract the tip; metal body
Papermate	Apex	Twist-extendable eraser
Serve	Swell	Twist-extendable eraser