Spring Assisted Knife Laws
What are the Legal Definitions?
Switchblade
A switchblade (also known as automatic knife, switch, or, in British English flick knife) is a type of knife with a folding blade that springs out of the grip when a button or lever on the grip is pressed. There are two basic types: side-opening and out-the-front (OTF). A side-opening knife's blade pivots out of the side of the handle (in the same manner as an ordinary folding knife). An out-the-front knife's blade slides directly forward, out of the tip of the handle. Many OTF (out-the-front) knives work with a dual-action mechanism that enables the user to extend and retract the knife in one press of the finger, with no cocking or priming action. However, some OTF (out-the-front) knives are single action, and require the user to manually retract the blade. A wide variety of blade designs may be found on switchblades, but the most common is the Italian stiletto style seen often in movies. However, the switchblade should not be confused with the butterfly knife (balisong), assisted-opening knife, or the non-automatic stiletto.
Assisted-Opening Knife / Spring-Assisted Opening Technology
A spring-assisted knife is a knife that when you push on the thumb stud to open it a spring takes over and propels the blade open. Spring-assisted knives make a great alternative to automatic knives. A Spring / Torsion assisted knife is a type of knife which uses a spring assisted mechanism behind the blade. They open by the ambidextrous thumb stud on the blade with a slight bit of pressure. They are commonly confused with switchblades, but have one main difference. While a switchblade can be opened usually with the push of a button within the handle, the user of a spring-assisted knife must apply slight pressure to the thumb stud and the spring/torsion assisted mechanism does the rest. Once the knife has been opened about one-quarter of the way (45°), the mechanism will open the knife the rest of the way. A/O knives are Assisted - Opening which are also Spring - Assisted knives.

US Assisted Knife Law
CALIFORNIA is the basis of all law due to it being typically the hardest and most progressive

California State Senator Betty Karnette of the 27th District is the author and who is responsible for Bill SB 274 and the legalization of Spring Assist Knives.

Senator Karnette realized that Spring Assist Knives are extremely functional tools by law-abiding citizens and that these knives serve an important utility to many knife users, as well as firefighters, EMT personnel, hunters, fishermen, and others who utilize one-handed opening knives.

Spring Assist Knives are intended to save lives while an EMT, Firefighter, or Law Enforcement Officer is trying to get you out of an automobile accident or any other viable problem. They have saved skydivers lives that had to cut parachute lines. They have saved countless fishing poles when a line is needed to be cut. They are also essential to handicap individuals whom happen to have one arm. Special Thanks to Senator Betty Karnette is in order because if it were not for her we would loose more of our rights in this country and also not have tools that are lifesaving instruments.

LEGAL LAWS & STATE STATUTES OF CALIFORNIA

...read the full guide with pics here

Thanks for posting this... but its also worth mentioning that all 50 American states as well as most major cities have unique and often complex knife laws that may define switchblades and assisted-opening knives differently than the definitions that Mr. Mayor posted. Plus, precedent from cases that have been tried in various jurisdictions may define what is and is not considered legal beyond the bounds of the written law.
I see not a functional distinction between assisted openers and switchblades. The practically universal laws in this country banning switchblades from reasonable use for normal people have most likely had a negligible effect on crime. Since these laws are not the sort of thing likely to be overturned by shifting public sentiment, "assisted opening" knives seem in my opinion to represent a "back door" which bypasses the intent of the anti-switchblade legislation on a technicality.
Of course these knives are very useful in everyday and emergency activities; then again, their proscribed cousins are just as useful. If assisted-openers fall into the view of the ignorant, yet judgemental, public in the manner of switchblades 30 to 40 years ago, we should regretfully expect to see new additions to our laws creating wider and more restrictive bans.
The only thing that most owners of any knife or sword can do is act maturely and try not to scare people. Scared people write letters- with knee-jerk proposals- to their legislators.