Gravity knives were popular back in the '40s and issued during WWII to some airborned units. It is a knife that you would hold by the handle end downward and with a push of a button, the blade would slide out and lock into place by gravity. You could use the knife with one hand to cut yourself out of your parachute if necessary. Here is a pretty good Youtube video on them:
http://www.youtube.com/watch?v=RZjXuiC8t1E