reg 1.172-1
(f) Electing small business corporations. In determining the amount of the net operating loss deduction of any corporation, there shall be disregarded the net operating loss of such corporation for any taxable year for which such corporation was an electing small business corporation under subchapter S (section 1371 and following), chapter 1 of the Code. In applying section 172(b)(1) and (2) to a net operating loss sustained in a taxable year in which the corporation was not an electing small business corporation, a taxable year in which the corporation was an electing small business corporation is counted as a taxable year to which such net operating loss is carried back or over. However, the taxable income for such year as determined under section 172(b)(2) is treated as if it were zero for purposes of computing the balance of the loss available to the corporation as a carryback or carryover to other taxable years in which the corporation is not an electing small business corporation. See section 1374 and the regulations thereunder for allowance of a deduction to shareholders for a net operating loss sustained by an electing small business corporation.
what this is saying is you use up one year of the NnOL carryforward period for the C-Corp for each year the corporation was an S-Corp
