The down arrow generally comes on for me when I'm heading up Afton Mountain once the battery charge gets down to one or two bars left. That's about when the boost indicator starts to cut back and the car begins to show serious lack of punch. I drop down two gears and everything's fine.
Basically, since the electric motor provides low-end torque, if you USE that torque long enough to drain the battery, the entire system loses low-end torque, so you need to shift down to push the engine into the high end, where it has lots of torque. It burns more gas, but then, if you are headed up a mountain, hey, that burns gas.
This is what hybrid design is all about. Each side of the hybrid offers what it has and when one side doesn't have any more, the other side kicks in. You still get the best gas mileage and lowest pollution anybody could get driving up that mountain.
Face it, most motorcycles get worse gas mileage than the Insight.