Rostichery is that like cornstarch as in cornflour?
Corn starch is a thickening agent you might keep on hand for making pies, for example, when you need to thicken up the fruit mixture so it doesn't get so runny that the pie crust is a soggy mess. I think the same principle is being applied for fabric--the softness of the jersey is being kept in check by the starch.
Just make sure that what you are using isn't corn meal, which looks like flour and is used for making cornbread and such.
Yup that sounds like what we UK-ers call cornflour...cool thanks!