Table per group is a new feature to allow larger collections/binaries/parts without slowing down. Also, included with this new table set is threaded releases. It makes the entire loop very fast without much effort.
How it works. For each group, it creates a separate collections/binaries/parts table and no longer uses the default collections/binaries/parts tables. Before switching, you should run:
php reset_truncate.php [true, drop]