One of the more obscure features introduced in JPA 2.1 and Java EE 7 is the unsynchronized persistence context. It's not too surprising that the community does not seem to discuss it much - it's really geared towards somewhat an edge case.
Prior to JPA 2.1, the entity manager is always synchronized with the database when a JTA transaction commits. This poses a challenge if you have a multi-step workflow (let's say in a stateful session bean/CDI conversation/JSF flow) where you want some data to be flushed to the database on transaction commit at a given stage of the workflow but not all - but you do eventually want to synchronize all changes at some other point in the workflow. Non-standard solutions like Seam and the Hibernate flush mode have been the answer in the past to get around this problem. This is now solved in JPA 2.1 via the unsynchronized persistence context. In an unsynchronized persistence context changes are not automatically flushed to the database on transaction commit. Rather, you explicitly join the transaction when you want changes synchronized.
Hantsy Bai does a decent job explaining this through code examples. By far the best explanation of the feature comes from specification lead Linda DeMichiel. Check out minute 11-14 in the short video below (if you are having trouble accessing the embedded video it is available here).
What do you think of the feature? Do you think it is useful or obscure? The JPA spec is the best place to get further details if this intrigues you.