Wikipedia
A gravity knife is a knife with a blade contained in its handle, and which opens its blade by the force of inertia or gravity. As the gravity knife requires gravity or spinning motion to propel the blade out of the handle, it differs fundamentally from the switchblade, which opens its spring-propelled blade automatically upon the push of a button, switch, or fulcrum lever. The main purpose of this opening method is that it allows opening and closing to be done one handed, in situations where the other hand is occupied. A major historical use has been in issue to parachutists to cut off their parachutes when tangled in a tree or in similar situations.
The gravity knife uses a button, trigger, or fulcrum lever to release the blade from both the open and the closed positions, and may use a side-folding or telescoping (out-the-front, or OTF) blade. While most military gravity knives utilize a locking blade design, other types may not mechanically lock open but rely instead upon friction to wedge the rear section of the blade against the interior of the handle. Factory-made gravity knives have various types of buttons, triggers, and fulcrum levers, which usually are used to release the blade from both the open and the closed positions.
Usage examples of "gravity knife".
Arquillo's handmade gravity knife, with which he had disembowelled three effete men - the mules of a rival drug dealer?
She recognized the type, a Balisong folding gravity knife if you hung around Society types like Chuck Barstow, you overheard endless talk about everything from broadswords to fighting knives, like it or not.
There was a Japanese gravity knife with orange handles beneath his pillow.
A flat gravity knife, available in most army surplus and pawn shops would be the best type available in regular over-the-counter buying.
Toward the rear of the counter a kid was demonstrating a gravity knife to a couple of his friends.
He was dressed in jacket and breeches of white linen, the little peaked fore-and-aft cap of his region, and a sash which nearly concealed the pepperpot pistol and pearl-handled gravity knife he preferred.
Deftly the detective jerked his arms back and cuffed him, patted him down, and came up with a gravity knife and a nasty-looking handgun.
I found two throwing knives, another stiletto, a very neat gravity knife, a saw-edged Bowie, strangling wires, and a small metal case containing various powders and vials of liquids which I did not care to inspect too closely.
Deuce pulled a gravity knife from his back pocket, did one of those quick wrist movements that spilled the blade open.