What is the "Unscope"?
In the FPS (first person shooter games) context, unscope often means taking a shot without crosshair on the screen. This is particularly the case with the AWP (sniper rifle) in Counter-Strike. If the player does not go into zoom mode, there is no crosshair on screen. The player therefore has no point of reference, and knows exactly where to point his weapon. But going into zoom mode when an enemy appears takes time, and as you know time is of an essence on multiplayer FPS game. Hence the interest of taking the shot unscoped ...without cheating, but simpy using a small trick;)
A well-known technique used is to correctly position the center of the screen on the target and take the shot. Simply place a dot in the middle of your screen.
Some do it in permanent marker, or use a small piece of sticky that holds its own without causing any damage (static electricity).
For the rest, it is only of good practice.