Tom Clancy's Rainbow Six Siege - How to effectively use Peekers Advantage.

The "peekers advantage" is a parameter in the game meant to diminish camper's effectiveness (that or to compensate some lag issue). It makes the player who is peeking (moving) to see the player who is standing still a few miliseconds before the later, in a "cornering situation". - Definition per google search engine.
Basically moving quickly in and out of cover increases your chances seeing your enemy before they do. Coupling lean and pre-firing corners will give you the best chances at landing your shots first.