An assisted-opening knife is a type of folding knife

Knife

A knife is a cutting tool with an exposed cutting edge or blade, hand-held or otherwise, with or without a handle. Knives were used at least two-and-a-half million years ago, as evidenced by the Oldowan tools...

 which uses an internal mechanism to finish the opening of the blade once the user has partially opened it using a flipper or thumbstud attached to the blade.

The first assisted opening knife was designed by Blackie Collins

Blackie Collins

Walter Wells Collins or Blackie Collins was an American knife maker who designed and popularized the assisted opening mechanism and various automatic knife designs within the art of knifemaking...

 in 1995 and was named the "Strut-and-Cut"; it was based on the strut of his Ducati motorcycle. A similar concept was developed years later by knifemaker Ken Onion

Ken Onion

Ken Onion is an American award-winning custom knifemaker based in Kaneohe, Hawaii, USA who invented the "SpeedSafe" assisted opening mechanism for Kershaw Knives...

 with Onion's idea based on a similar mechanism in his Harley Davidson motorcycle. Onion applied for a patent on his design in 1998.

When the knife is in the closed position, the blade is held in place by means of torsion springs and an additional blade lock (optional). As the user applies manual pressure to the thumbstud to open the knife, a mechanism such as a torsion spring moves along a track in the liner and rapidly rotates the blade into the open and locked position.

Although commonly confused with switchblade

Switchblade

A switchblade is a type of knife with a folding or sliding blade contained in the handle which is opened automatically by a spring when a button, lever, or switch on the handle or bolster is activated A switchblade (also known as an automatic knife, pushbutton knife, switch, Sprenger, Springer,...

 knives, there is a difference. While a switchblade can be opened automatically with the push of a button

Button (control)

A push-button or simply button is a simple switch mechanism for controlling some aspect of a machine or a process. Buttons are typically made out of hard material, usually plastic or metal. The surface is usually flat or shaped to accommodate the human finger or hand, so as to be easily depressed...

, the user of an assisted-opening knife must apply some manual pressure to the blade at the thumb stud or flipper.

Because the blade does not open by the push of a button or by force of gravity, the assisted opening mechanism is not considered a switchblade. Once the knife has been opened about one-quarter of the way (45°), the mechanism opens the knife the rest of the way.

Other trade names for assisted-opening knife are: Forward Action Spring Technology, A/O Knife, Torsion Assist Knife, Assisted Knife, Spring Assist Knife, Spring Assisted Knife, Quick Release, Quick Draw, Alternative Automatic, Outburst, SpeedSafe, Blade Launcher and the Semi-Auto.