when fixed costs decrease, what does this do for sales?

Fixed costs are expenses that dont change, such as rent expense, phone expense (if same month-to-month), etc. These expenses do not have a direction relationship to sales.

With variable costs, such as advertising expenses, you can possibly identify a relationship. Increasing advertising expense, increases sales.

So to answer your questions, when fixed costs decrease, this does nothing for sales. It does however increase net profit.