Use the trackball to move.
Swiping your finger across the screen draws a beam of energy which will hit any enemies near it when you release your finger.
Killing multiple enemies in a single shot rewards you with a score mulitplier.
The longer the line you draw the more ammunition you use up.
You don't have to draw a straight line with you finger, try different patterns to maximise your kills.
Different enemies take different numbers of hits to kill, to maximise your score try to kill as many enemies in one shot as you can.
Some enemies are more dangerous than others, it might be worth attacking those first.