I’m putting this out as the functional definition we’ve used at Fresh Pond Monthly Meeting, New England Yearly Meeting
Threshing sessions are a form of Quaker worship and practice intended to help a community prepare for a business meeting, especially one which involves a decision on a difficult or potentially controversial topic. Threshing sessions are held as part of the process of preparation for individuals as well as the meeting. It is a time for individuals to raise up concerns and hopes that they have in regards to the issue being threshed.
Although it is clerked and a Clerk may at times test their sense of the Meeting, it is not a called Meeting for business and no binding decisions will be made. Even if the sense of the meeting seems clear at the end of the threshing meeting, Clerks will only be taking notes and making a record of where the group has reached: the decision will be left unmade until the next full Meeting for Worship with a concern for Business. This is a time for sharing information, extended exploration, and clarification as to what needs to fall away and what will remain as the centering focus of any later business.