Instant raising and lowering sectors
Instantly raising and lowering sectors are a Doom level editing trick that exploits a bug in the way a sector detects which floor to move up or down to that causes them to change heights instantaneously with no detectable movement animation. It is caused when a linedef type calls on a floor to move down toward the lowest adjacent floor when the lowest adjacent floor is above it's current height, or when a floor is called to move up to the next adjacent floor, when all adjacent floors are below. When a floor does not have another floors to move up or down to, it instantly changes height to the closest floor in the opposite direction that the line action tells it to do. This behavior also works with ceilings and doors, causing instantly moving ceilings, but in general, are less malleable to be exploited for the level designer's benefit than floors.
Instantly raising or lowering sectors can be used for a variety of effects ranging from gameplay enhancing scenarios, to optical illusions. The most common use of instantly raising floors are for instantly appearing monsters. This is usually done by placing monsters in a sunken pit in a floor, fixed with missing lower textures so that the surround flat bleeds over the top of the pit, effectively hiding it. The player can be lured into an unsuspecting trap, tripping a line that lowers the floor to the nearest floor, causing the monsters to jump scare the player with no plausible warning.
Instantly raising and lowering sectors can also be used for illusions such as animating a swinging door or gate, breaking glass or a destructible wall, custom switch textures, pushing a crate, or simulating a 3D bridge that you can walk over or under.