I tried the feature briefly with Pre release version. As I remember it the PDM creates new files for the files that you want to branch, and they are routed to workflow as a "fresh" file.
Once you merge back to the original file, you need to check out the original file or atleast need check out rights, not sure if you needed to manually check out or not. If you don't have check out permission, you need to progress the files to a state that does allow it. For example trying to merge over approved file, you might need to first start new revision to get the file "In Change" and then merge the changed part over.
To me it seemed that the branch/merge feature does just automate the manual process of saving a duplicate file to test changes before committing to a new revision for an approved part, and also the saving over the original file with the duplicated file. Nothing groundbreaking but it keeps track of the files so they won't get lost.