My girlfriend and I helped with the purchase of a 1991 Legacy L that had the same feature. During a vigorous test drive (
) I saw the light come on. I think the car is just saying, "Hey, I'm going as fast as I can!"
There's also a manual button that allows you to start the car in 2nd gear, like cheap traction control I guess. Do you have that light as well? Do any later models have either of these features?