Pie-ing an entryway
Pieing is simply the process of moving carefully and deliberately in a fashion that allows you to see as much of an area as possible from the outside, by moving from one side to the other, before entering it.
Pieing a room allows for you to visually clear everything except for a corner or two, which allows you to enter and immediately focus on the danger areas (i.e. uncleared corners) without having to do a full sweep of the rest of the room at the same time.