I have created my own video tutorial about overblowing, which I believe takes a unique approach that might help beginners to overblow. In this video I tried to teach overblowing kind of like people are teaching how to draw bend. I am talking about sounds you should produce within your mouth to get overblows and even to be able to bend overblows.
I already overblow, but I see where you are coming from with the hiss sounds. This will help some people. The only thing I'm not sure about is thinning the airflow. You do pull the tongue back and make it fatter, but this, to me anyway, redirects the airflow more than anything...especially with a big, open throat embochure. It feels like the airflow hits the roof of my mouth when I overblow. Great video.
At some point I was sure that overblow mechanics where that the air was entering the harmonica in an angle which causes a dragging force to close the blow reed making the draw reed the easier reed for the air to flow. There is even an academic paper on the subject which that was the conclusion. If that was what actually happened inside the chamber than doing an overblow with the harp up side down with the same embouchure would not work but it does.
What I feel that is happening is some kind of a whistle effect which vibrates the air in such a way that the draw reed is resonating... any way just a theory...
I feel the air hitting the roof of my mouth too... With the 1 overblow I got the idea of the pitch of the hiss cause to get a clean one overblow I really have to make a low hiss and I think I have a very clear 1 overblow compared to some the videos out there... even Jason does a lousier 1 overblow (in one of his videos)... although I would trade my 1 overblow for all the other things Jason can do... :-)
I have created another video where I am trying to help people overblow and this time I am showing that overblowing is not about the angle the air gets into the harp but rather the air density which is caused by the back pressure of the thin passage created in the back of your throat.
So here is the video:
I am planing on doing a few more of these videos try to share a beginners view...
