It's no secret that General Motors is spending lots of money on incentives to keep its plants running full tilt. The automaker's much-publicized zero percent financing deals powered a 24 percent sales gain in July in a market that was up a comparatively low 8 percent.

But according to Business Week, GM has another trick that has brought in plenty of buyers: Discounts offered through its GM

Mastercard. The incentive program has earned a quiet but powerful windfall for GM, so successful that GM is planning to cut back on it.

But the retreat won't take effect for another year. In the meantime, thousands -- if not millions -- of potentially eligible car

buyers will take advantage of the plan, giving GM a big boost in its bid to gain market share, according to Business Week.