It's done very quickly, and demands a lot of skill to get really good at - but a heck of a lot easier to do than Quick Scoping. Another term used in sniping is 'Drag Scope,' which means to hard scope in, and quickly drag your cross-hairs over your target and shoot. On the opposite end of the spectrum is the term 'Hard Scope,' which means to eye your target through your scope, line up your target with the cross hairs, then firing.
On BO2, the term quick-scope changes to describe actually scoping in, and as soon as you see your target, you fire. Since BO2, quick-scoping as it was became unusable since bullets would drift inconsistently every which way. 'Quick Scope' is a term that used to describe not scoping all the way in-you would fire before actually seeing your target while the scope sighting was still black on earlier games such as Modern Warfare 3. A skilled sniper, coupled with the ability to 'Quick Scope,' could actually run away with the game.
