When you have a screen door without a screen, you open the sliding window and attempt to secure it open. This is used if you don't yet have a cat door and want to let your cats in and out freely.
Sometimes the window that you thought was secure can fall, mid jump of the cat, and provide you with two kitties instead.
Disclaimer: This has not happened to our cats, although close calls have occured. This redneck contraption is what we refer to as a kitty guillotine.