In project-based projects, freelancers don't get paid hourly but for the completion of milestones. A milestone is a section of a project that the freelancer and client have assigned a certain percentage of the overall budget. This section could be as big or as small as they want.... it could be 1% of the budget, or it could be 100%.
The freelancer can suggest a certain fee for a certain part of the project by submitting a milestone proposal. If the client accepts the proposal, that means he's committed to the terms the freelancer submitted - that he will pay what the freelancer offered in exchange for finishing that segment of the project.
When the client accepts the proposal, he is immediately charged the value he promised.