Phasor Zap presents the player with a purple-tinted starfield that represents the view from the player's spacecraft. Enemy spacecraft appear from the edges of the screen and move straight across it, and it's up to the player to use to the paddles to focus four "phasor" beams onto each ship to destroy it, earning points. The player begins with a limited amount of phasor energy, one point of which is consumed each time the player shoots. When the amount of remaining energy reaches zero, the phasor is depleted and the game is over.
Source: Wikipedia, "Phasor Zap", available under the CC-BY-SA License.