How do they establish curves for this test?

I am wondering how the curve works for SAT subject tests.

With the regular SAT that is fairly easy because a the test takers are generally representative of most school students. However (at least in my state) the SAT subject tests are taken by the rare braniac who wants to go to difficult to get into schools. I would think this would skew results heavily towords higher scores causing a less leniant curve, however the opposite. SAT subject tests seem to have a more leniant curve then the general SAT.

Where do they get the data in order to fit SAT subject tests onto a curve?

Each SAT is not graded on a curve. I believe that there are also a few questions on each test that are not graded but are being tested for future dates. I would worry about other things, I think. ETS isn’t perfect, but it is staffed by tons of people who realize that if these tests don’t have integrity, they are useless.