This is the same game you can find in the games section of StencylForge (search the games section for "Demo-" to find all of my demos). It is designed to teach others how to do the coding, so please feel free to take a look at it :).
As for the game itself, you can use the arrow keys or WASD to move and the mouse to shoot. You can also press Z to toggle on/off an auto-fire mode. Press space to enable a temporary power-up (OHKO the asteroids). Enjoy trying to find the ends of the scene... even I can't find them ;).