Its a bit hard to explain, but for each .waw sound file you will see at the top of the soundset file, you can set them a preset curve(included on soundutil, for example for electric engine sounds) or you can set them manually. So if you set them manually you need three values(gain/power/pitch) that needs a curve given by the list of { speed, value}. That's used for example when you add a sound that only plays at high speeds or similar.

I know the basics.

What is the 2nd entry starting with
for?

I don't see it anywhere else, in no soundset from any mod.

Its a value you can add for the power, that you can understand as the base level volume for that soundfile

In this case is used to have a higer volumen at top speed that the volumen that has the soundfile
That why the last pair of values is { 1.0(Speed), 2.7(power) }

Where did you get the syntax from?
I did not find any tutorial or wiki that explicitly tells me that I can use a 2nd array inside the same parameter starting with an *