Thrust, turn, shoot. The ship keeps whatever speed you gave it until you point the other way and give it some more, so most of the first minute is spent drifting backwards into the thing you were aiming at. Shoot a rock and it splits into smaller rocks that move faster, which means clearing a screen makes the screen more dangerous before it makes it safer.
There is no story and no upgrade. There is a screen that wraps at every edge, a small number of lives, and a score that goes up until it stops. It is the 1979 cabinet, and the reason it is still worth an evening is that the momentum never stops being the thing that kills you.