Undoubtedly an extremely controversial topic, we used to work it routinely. It involves the concept of shooting an adversary directly through an interior wall.
For example, your partner is moving into a room and takes fire at the door from the extreme left side of the room. You bring up your rifle or pistol and shoot from the area adjacent toe door to the extreme left area of the apparent room at face/upper chest level, then lowering your muzzle to where a man may go if he takes a knee, bring your shots back toward the starting point.
Think of writing a long "C" with your shots.
What is needed -
1). Certainty that the adversary is in the room alone. The only thing worse than missing a shot on a bad guy is shooting a non-combatant.
2). The ability to penetrate the interior wall. This is easily done with anything anyone would care to carry on a sling or in a holster. Note - Furniture may affect performance.
3). An understanding of human nature. When shots are fired, most people will drop their profile.