The more you spend, the higher it rises in the "promoted" list, which is a unique category. Also, there are "promoted" lists in each category. For $15 and $16 I was able to put two of my posts at the top of the promoted posts in the story and fiction categories for several hours today. One rose about $100 and the other went from a few cents to over $300 after promoting it. Since upvotes weighty enough to add cash, also add time to the payout schedule, I imagine the promote button does also.