Gravity knives were invented to solve a simple problem that ultimately helped save lives. During World War II, soldiers on both sides of the conflict would come to use the blade during paratrooper drops. All too often, the jump and landing wouldn't go precisely as planned, and paratroopers sometimes got tangled up upon landing. With a gravity knife, access to the blade could be gained with the simple use of one hand and a couple of fingers at most. So, this device's highly useful nature often made the difference between being stuck or stranded and getting on the move again to execute the mission.
The first German version of the gravity knife was first made by Luftwaffe Fallschirmjäger-Messer, who pioneered its design. Then George Ibberson & Co. of Sheffield, England, produced another popular version a few years later. This version was more or less the same design as its German counterpart, although the British version sported either a plastic or wooden handle, depending on the design.
Since the end of World War II, the knives have been both a convenience and a headache for law enforcement. It's often difficult to distinguish a gravity knife from some of its "cousins" among blades, especially the switchblade and the butterfly knife. Because of this challenge and the apparently subtle differences between them, different laws apply in various U.S. states, counties, and municipalities, and this sometimes makes determining the legality of a give knife difficult wherever you happen to live and work.
In New York City, for instance, the NYPD is constantly on the lookout for knives of this nature. Even if the user mainly uses their knife for work-related purposes, it's still conceivable that this individual may be arrested for 'criminal possession of a weapon' if found with a gravity knife in NYC. This could occur in a situation as simple as being subject to a common search on the subway.