Peeker's Advantage:

Peeker’s advantage is the edge a player can have when swinging out from cover, because the peeking player may see and shoot an opponent before that opponent appears to react on their screen. In Counter-Strike 2, it matters in duels around corners, tight angles, and entry fights where timing and positioning decide who lands the first shot. Players often account for it by clearing angles carefully, holding off-angles, or using utility to reduce how much a defender can rely on reaction time alone.

Related: Pre-Fire