It's primarily an intimation about the tendency of old arcade games to have a strong vertical dynamic to their design. In many such games, the player occupies the bottom half of the screen, and the CPU opponent controls the top half. Every five seconds, the game swaps the bottom-screen protagonists and top-screen antagonists of several games, demonstrating that the vertical dynamic inherent in these games allows such mashups to be both playable and interesting.
Play the game using only the mouse!
This mashes up the following games (don't read if you don't want spoilers):
* Space Invaders
* Missile Command
* Pang (a.k.a Buster Bros)
* Crystal Quest (admittedly not a vertical game, but included for variety).
There's no sound because I didn't feel like it.