At the end of every phase, the contractor will click a button inside their project portal, telling Ditto that a phase of work is complete.
This triggers a notification email to the client, telling then that the phase is now IN REVIEW. (They'll see the phase state change in their project portal, too.)
Once a phase is IN REVIEW, the client has four days to respond. If they do nothing, Ditto will assume that they're happy with the work, and the payment will be released to the contractor.
If the client is not satisfied with the contractor's work for a given phase, they have the option to refuse to release payment for that phase. They do this by clicking the REJECT button next to that phase in their project portal.
When the client clicks this button, they will be prompted to share feedback with their contractor. We expect that they'll provide information about what isn't quite right and request changes to improve the work that was just delivered. This feedback is not shared through Ditto - it happens directly between client and contractor however they typically communicate (email, Slack, phone, etc.). Ditto does send a notification to the contractor indicating that the payment was not released and that more work (or at minimum, a conversation) will be required.
(All of this is outlined in "legalese" as part of the Ditto Terms & Conditions, which every user of Ditto must agree to in order to sign up and use Ditto.)