Some people confuse peeking and pieing with limited penetration. They're based, partly, on clearing from outside the door but with emphasis on dynamic or immediate entry. Peeking is taking a 'snapshot' of an area by poking your head out and then bailing to concealment. It isn't made for fighting from the door or holding threat angles. Pieing is a semi-circular motion at the door that aims to enter immediately. The operator has limited opportunity to engage unless they distance themselves from the threshold before breaking the plane of the door with flank exposure to blind corners. Often, due to action-reaction or reactive-proactive, a moving shooter who is putting themselves in the middle of the door as they pie from frame-to-frame, is at a tactical disadvantage against a prepared defender.