I'm not sure... when I put in all my info on the first page, it takes me to a list of prices with various companies. Then I click on the left top section of the list or the very top "Name Your Own Price. Start Now" button. At the next page, I reenter all my information for what I want, and then at the bottom enter a price. At the next page, it'll confirm all of your details (and maybe say that your price is not likely to be selected, which mine said when I did it, but was still accepted, so I guess that's not always the rule) and then you enter all your credit card info. At THAT page is when you click the "buy" button at the bottom, which says that if a company accepts the price you entered (and they confirmed on that screen), you will immediately be charged for the rental and it's not cancellable and all that. After that page is when they'll tell you if it's accepted (or if you need to check your email).
I hope that helps - I don't use Priceline too often, so I'm not 100% sure on everything.