The two lanes right above the pops are bonus multipliers. When you light both, the bonus multiplies. Use the flipper buttons to switch the unlit one just before the ball rolls over the switch (like a lane change feature) so the ball lights it. I think the most I've gotten so far is 24x bonus. It is per ball, not per game.

It's likely just switch hits like most of its contemporaries.

You have to light both bonus lanes. Once they're both lit, bonus accrues & the lights go out so the ball can relight them again. The bonus is collected at the end of each ball. Bonus is reset back to zero at the beginning of each ball. Ideally, you want to keep the ball in the pop bumpers so they keep popping the ball into the bonus lanes. You have to be quick with the flipper buttons so you can change lanes as the ball rolls through the lanes.

For the second time in this thread, the OP is asking about the bonus value itself and NOT the Bonus-X multipliers.

This is something that is largely missing from later games. If the bonus appears in the status reports you can play with the glass off and compare. I'm sure at some point someone will take a peek at the code itself to determine this but you could verify pretty easily this theory by trying it.