In most self defense situations, you'll have plenty of time to open your knife in a more conventional way. It might be nice to practice quick openings, but do it when you're wide awake and keep your fingers out of the way in case the blade doesn't have enough kinetic energy to snap open. Another thing to consider, the worst thing you can do in an encounter is flip open a knife in a way that could be perceived as aggressive later in court. Even if you're justified in using a knife for defense, you don't need witnesses saying, "He snapped open his knife just before the stabbing."
Four-inch bladed knives can easily be mastered for quick openings, but 5- and 6-inch bladed knives need practice. I love Cold Steel because they produce such knives, and they're high quality. I was able to get a Rajah I knife (several, actually) when it was discontinued several years ago. Astounding knife. I also love the Voyager and Vaquero models of knives. Unlike the newer models, they were light weight, strong and more easily carried.
I remember the first time someone showed me a 6-inch Vaquero with that cool, serrated recurved blade. I was not interested in going against someone a knife like that!
So if you want to flick, practice. Even 6-inch bladed knives can be easily flicked with practice. The heavier these knives get, however, the tougher they are to flick. But like anything else, start out slow until you get the movement down, then pick up speed -- and keep your fingers out of the way of the blade. The only time I was seriously injured was as a kid. The blade stuck in the wood, then snapped closed on my thumb when I sought to extract it. I didn't know there was that much blood in a person!
