A portcullis moves between the floor and 32 above it. Only one in a room moves at a time: $5BAF counts the one moving, and while it is non-zero nothing here happens. A portcullis anywhere below the top starts to rise at a random moment, on average once every 32 frames. One at the top drops at once for its first four falls after the room is entered (counted in $5BB0); after that it too waits for a 1-in-32 chance on each frame. Setting bit 0 of the type makes it type 9, handled by upd_9 while it moves.