Crouch Cover is a technique for using cover that conceals you when crouched and allows you to attack when you're standing.
Original Author: Pimgd
When: After firing, during reload.
What: Crouching and reloading.
Why: Reloading is when you are vulnerable. We all know that by now. Your goal during reloading is to get hit as little as possible. Crouching is a good way to reduce the area you can get hit in.
How: Take Tower, Swat side, with the fence. It has boxes. If you stand on those boxes, and aim your gun over the fence, you can shoot people heading towards the roof. During reloads, you can crouch to prevent being shot.
Bonus: In 3rd person, you can use this technique to stay hidden and strike enemies without them seeing you.
How to practice: Find a spot that conceals you when crouched, but shows your upperhalf (or just head, depends) when standing. Like Tower, SWAT side, with the fence, on the boxes. See how crouching hides you and standing allows you to shoot. Then try firing, and crouching the moment your magazine is empty.