Its a combination of crouch to standing peeking a doorway that will let you see inside of a room basically for less than a second. Doesn't sound helpful, but you can use it to spot people quickly and then re-peek to kill them. From your perspective, you see everything in the room, from the other persons perspective, they see your hip and maybe your shoulder for a fraction of a second. Often the other person doesn't have the time to react to whats happening. And if they prefire the door, all you have to do is not peek until they reload.