The main idea of it is you should take the game field by cutting it off the top canvas. There are enemies which will try to prevent you from doing it. If an enemy touches you or your cutting line you lose one life. You also lose a life if you go across your own tear. When you lose all of your lives the game is over.
There is a particular beauty of this version of the game. You can customize the game field, the enemy images and the chopper images, even the levels. Everything is simple. The only thing you need is the industry standard (JPEG or PNG) images. Then you just overwrite existing ones in the "Images" directory and that's it. You have completely customized game with your favorite pictures. If you want to go further and customize the levels you have to open the LevelsInfo.xml in any text editor and adjust desired parameters. The initial game field layout is in the InitialField.dat. Editing the initial layout is probably the most difficult part. You are going to need some sort of a hex editor. In the file you'll find a field 80 by 60 bytes. 0x00 - means top canvas, 0x80 - means bottom canvas.