The goal is to advance two parallel sets of inserts, green and amber, so that the lit inserts are next to each other. For instance the second green and the second amber are both lit. On comes a red light and the center standup target awards another ball. Not as easy as it looks. You advance them via the green and yellow pops. Some games are a real beating. Some games, the flow is just there and you hit three add a balls in quick succession.
You also get extra balls for scores and for making the top rollovers. It's all about the extra balls.