The first time I heard of it was about a year ago. I used to carry a derringer because I could make it disappear, but it seems easier to make a keychain can of pepper spray and a folder look very natural and unobtrusive. The idea is to use the pepper spray first and then open the knife. If you still need to do something, your attacker is partially disabled. The last confrontation I was in was a bar fight. Some drunk asshole took a swing at my friend out of nowhere, and about ten of his friends stood up to take his side. A can of pepper spray would have partially disabled all ten, without killing anybody, and without a loud bang. As it was, we all went to it with fists for a minute. Nobody really wanted to fight so it wasn't that bad and it was over quickly. But having a can of pepper spray would have made that fight very one-sided in a hurry. I would have used the pepper spray, I didn't use the derringer. The pepper spray can has 25 "applications" to my derringer's 2 applications. I think the silence plus the lack of deadly force plus the backup of a decent folding knife is a pretty good combo, maybe better than a gun.