Definition any knife with a blade that pops out, at push of a button, is considered as an automatic knives or switchblade knife (same thing). Thats the law. One can go to jail for carrying such thing without authorization. Again, as far as the law is concern, it has to be a button or a concealed/hidden firing system, not a rail, stud or whatever else. Some automatic knives, such as most of the OTF type (stands for firing Out The Front) use a slider instead of a button, and do not fall under that mentioned law.
Its a very good idea to check with local authorities regarding knives, any and all knives.
Any knife can serve as a weapon and it does not have to be a switchblade knife. Cops usually can tell you about particular regulations in their state, county or city.
Safety lock - holds the blade locked securely, so it will not open accidentally. May cause a loss of critical time in tactical situations, as it needs to be disabled prior of firing the blade; it is also almost impossible to operate with gloved hands.
Prominent button - sticks out of a handle. Has the advantage of being easily felt without looking; its disadvantage resides in the necessity of a safety lock, in order to prevent an accidental opening, while in ones pocket.
Flush button - does not stick out, reducing substantially the need for a safety lock. Its extremely rare for that type of automatic knife to open accidentally. It might require some rehearsals to find the button blindly. Pro Tech and some other manufacturers place the flush button in a concave depression, so one can still feel it blindly.
Concealed button hidden in some area of the handle, usually, that one has to push or slide or whatever in order to fire the blade. Most of these types of automatic knives also open manually, like any other folding/pocket knife, and they really look like one too, in most cases. Its the best system to increase the time it takes for a youngster to figure out what it really is and how to open it.
Slider as said above, its used with all (as far as we have seen) OTF types of automatic knives. It has the same advantages as the prominent button and it will not open accidentally by itself. Not used with any other type of automatic knife as far as we know; probably for technical reasons.
Hope that this helped. You may mail us firstname.lastname@example.org, for any additional information.