At each level, you have to destroy a certain number of diamond shaped extraterrestrial (I assume) ships. Once in a while, you will hear the sound of a missile incoming. When that happens, a good idea is to perform some evasive maneuver to get the heck out of its reach, in other words, to go backwards and sideways.
Once you have cleared the level, you have to go through some kind of (black) portal and face ... the freaking spheres. You can’t tell how it feels like until you have actually experienced it: it’s fast and furious. Once you’re done with the spheres, you are at the next level and you have to do the whole thing all over again. It gets harder every time of course.
As you can see, I have failed miserably at avoiding all the spheres. Not the first time I am afraid.