Allows the promotion amount or percentage to be negative. Both type of promotion amount can be entered in the Admin Workbench. Incase if you enter negative promotion amount then it appears as positive in the product catalog and shopping cart.
If you enter positive amount it will be applicable as negative that is discounted from product price. This feature is very useful as default Magento does not allow values less than 0 to be entered as discount in Admin Workbench.