I have never done the FIRE harden method and I don't think it is a good way because it's hard to see what's going on.
Clay Hays has some awesome Videos out there.
The best way is to use a heat gun like pat said.
When you use a chop saw and you have that tri-pod stand with a roller on top to hold the other end of a board up.
Clamp or tape your heat gun to that over the belly of the stave and work your way down.
I would say 4 to 6" above the stave.
This is Ash I did years ago.
The few light colored areas is where I scraped it some with the finish tiller.