Glossary
Peek
In one line: Stepping out of cover to look at (and shoot) an angle or area.
What it means
Peeking is the most common move in the game: you are behind a wall and lean out to look. Every peek is a small bet: you gain information or a kill, but you expose yourself.
There are many named peeks (wide swing, jiggle, shoulder, double). The shared rule: every peek has a purpose (see, kill, bait a shot) and you stand still when you shoot. Repeating the same peek on the same angle after a miss is a gift to whoever is waiting.
An example
You are behind a box and hear an enemy on the other side. Instead of waiting, you take a short peek on the right, spot them, stop and shoot. Had you missed, your next peek would come from the other side.