You've give up far too early, the definition is in the article: "Courts have interpreted that to mean that any knife that can be opened with a “wrist flick

You've give up far too early, the definition is in the article: "Courts have interpreted that to mean that any knife that can be opened with a “wrist flick” — a movement something like what a fan dancer might do — qualifies as a gravity knife. A long list of court cases have turned on exactly that question, which seems straightforward enough."