AKA Engine/Engine Piston/Crusher, this effect causes the ceiling to lower and raise itself repeatedly. I call it a Piston because it's only one word, and because it's easier to form an image in your mind of a Piston than an "Engine".
Create the piston sector.
Place the following sprites in the piston sector: S[0,25]
(Sp[0,Speed]). The vertical position of the S
determines the piston's initial vertical position (I usually just leave it on the floor).