At the beginning of a project, we'll work with you to develop a list of priorities for us to work towards. You'll choose an iteration duration — typically no more than a few weeks — and a maximum capacity to cap your out-of-pocket expenses for each iteration. At the end of every iteration, we'll provide you with what we've completed so far and an invoice for time and materials.
What happens next is up to you. Many clients will decide to modify their priorities based on the results of the previous iteration. If everything is perfect and you just want to keep going without changes, great! But if you're truly unhappy with our work — something that we hope is unlikely — you can walk away after any completed iteration. You're not obligated to start a new iteration at any point.
All of the parameters can be fine-tuned in between iterations, and the schedule is as flexible as you need it to be. For example, you may need to temporarily halt the project while you get more information. Or decrease the iteration time and switch around project priorities for an extremely experimental feature. No problem. And if you've discovered that what we provided last iteration has all the functionality you need, and the project is finished, that's no problem either.