Whenever an enemy fighter gets past you, it will fire on your ship. You will see the screen flash and shake if they score a hit. Watch your shields - when they go, so do you.
You can pause the game at any time by hitting the 'P' key. Hit again to resume. Use the 'M' key to toggle sound on and off.
You will need a fast machine to play this game, recommended at least a 200 MHz. It helps if you use a browser with a JIT (just-in-time) compiler for Java. Internet Explorer 4.0 seems to work best. Turning off the sound effects may also increase game speed.