I will rewrite the following description as needed.
Flow extension provides a discussion and collaboration system for talk pages. When the Flow extension is installed and Flow is enabled on a page, the page becomes a Flow board, i.e, the content model property changes to 'flow-board'. If Flow is installed along with its dependencies, the thank link for the corresponding user is available under comments on the Flow boards, clicking which will have the same effect of sending the user Echo Notifications. The corresponding class which needs to be implemented in pywikibot, along with proper testing, is ApiFlowThank (which extends ApiThank). This API is different from ApiRevThank, as it is called by action=flowthank, with parameters postid and token.