"Gravity knife" means any knife that has a blade released from the handle or sheath thereof by the force of gravity or the application of centrifugal force, that when released is locked into place by means of a button, spring, lever or other device.
Colorado Criminal Code: Section 18-12-101
Gravity knives are a rather neat type of knife, that are unfortunately illegal in many places. While the legal definition above could include many sorts of knives with pivoting blades, such as butterfly knives, a true gravity knife has a blade that slides out of the handle.
The blade runs along a track down the center of the handle. While not in use it is prevented from slipping out by a catch. By holding down a catch release the knife can be "unsheathed" with a flick of the wrist, or simply by pointing the blade opening downwards. Letting go of the release button locks the knife out. While there is no spring to propell the knife out of its handle, there is one to make sure that the catch is forced into the locked position while not held down. It is this spring that is the downfall of most old gravity knives.
Gravity knives are uncommon, and the only notable example of their use in a military capacity was by German paratroopers in WW2. These knives had wooden handles, and were equipped with a thin spike, like many sailing knives are today for the purpose of untying knots. They had a blade that was sharpened along one edge only, and of about 4 inches in length.