Feathering is a quick short back and forth motion. Maybe 1/2" or so. It's a fine tuning at approximately the apex of your stroke. Practice strokes are the classic full back and forth. You can feather, then practice stroke.
It's pretty hard to explain with words.
Here's a spot where he is doing feathering. I've not watched the video so no idea on it's quality or message, but this little snippet shows feathering. Mine looks a bit different, but basically the fine motion makes it easier to see if something is off. A full on practice stroke may be harder to see if you're off a little bit.