The idea is that you're smoothing out your buy in price because you're buying in a the average of all the prices. If you put $10,000 in on each of 9/1, 9/8, 9/15, 9/22, and 9/29, you'd end up paying closer to the average price in all of September than if you put in $50,000 on 9/1. Of course, as jonhohle states, if you expect the market to go up, you're losing a bit of buying power across the period you are DCAing in exchange for decreasing your risk.