Thanks for the help!

Potato starch (or potato flour or cornstarch) has a low protein content and is gluten-free, so it's added to American flour to help make our baked goods soft and tender and much less chewy, without losing any structure and without affecting the taste of the product.

If you don't have potato starch or potato flour, substitute an equal measure of cornstarch. If you have neither corn or potato starch, use all flour (in your case, 1 3/4 cups) and replace two tablespoons of the butter (or eggs if the recipe doesn't include butter) with two tablespoons of shortening, such as Crisco, or lard.

Potato starch also hold moisture in the bread so it doesn't dry out as quickly and ti helps produce a finer more tender crumb. It also feeds the yeast which produces another flavor to the bread..

I have use a few, mostly Scandinavian, recipes that call for potato starch. There are a lot of kosher-for-Passover recipes that also rely on it. Cookies and other baked goods made with potato starch have a really delicate texture.

Here's some information: http://www.ehow.com/facts_5664786_potato-starch_.html

In bread baking, mashed potato added to the dough produces a nice texture. My everyday bread recipe includes 1/2 cup of instant potato flakes. I realize the starch is a different thing, but I'd venture a guess that it's for texture.