The game fights back! Defend your green guns (which count as Bonus X) against attacking aliens (drop targets).
Your guns/bonus x are lit by making the various green-light lanes around the playfield. Yellow targets and lanes spot your attackers.
The attacks move down one row every few seconds. Any at row 4 for too long will KILL your guns (and bonus x!), meaning you must relight them
Bonus X can easily be worth 20K+, so make it a priority to take out attacks that correspond to your already-lit guns.