I took a felling class put on by Husqvarna. The instructor was Ron Hartill, you may have heard of him. If not, he was a 7 time World Champion Lumberjack back in the 70's. He know his logging and felling.
The popular way to fell a tree is flat, down, back. And that is the way I was first taught, until Ron pointed out some good things about Down, Flat, and then Back.
Ever since then I have always (when using a TRADITIONAL notch) put in my 45 degree cut in first. Then lay my saw in and do the flat cut. The point of this is to be able to glance inside of that 45 cut to see your cuts match up perfectly. That way you don't mess up your notch. I watch others use the popular way to do this and see them cut, re-cut, and then stop to re-cut again.
I can see that when using a HUMBOLT notch it is best to start with the flat cut first. Perhaps some are getting these two confused?
At first I was hesitant and not so well with this method, however, I bacame well with this. My cuts match up all of the time and my notches are beautiful, thanks to Ron.
See attachment of the beautiful hinge wood from a Siberian Elm