This major undertaking was discussed by Matt Ahrens on our recent ZFS episode. How it works:

The feature reflows existing data, essentially rewriting it onto a new arrangement of disks – meaning the original group plus a newly added disk. In so doing, a new adjacent chunk of free space is created at the end of the logical RAID-Z group and thus at the end of each physical disk.

With a disclaimer:

While all capabilities of this feature have been implemented and all tests so far have been passed, there are still a few loose ends to tie up. Specifically, there is some code cleanup to do, some verbose logging to remove, some code documentation to write, and similar relatively minor tasks. We aim for this to be integrated by Q3.