During a normal draw bend both reeds can participate in making the lower pitched bent note. The draw reed lowers its pitch and sounds as a closing reed note, while the blow reed raises its pitch and sounds as an opening reed note, actually dominating the sound. A full bend sounds almost exclusively from the opposite reed. In other words, a full draw bend sounds from the blow reed, and vice-versa for blow bends The point is, a reed sounding as an opening note with a higher pitch is not unusual on the harp--it is normally just hidden within ordinary bends! Overbends aren't any more stressful on the reed than normal bends, if they are played properly.
The term "overblow" was coined by Howard Levy when he asked a brass player what they called "created" notes that aren't natural to the instrument's tuning, and the term has stuck. Don't be mislead from the name into thinking you have to blow or draw hard to get the overbend note. It's a matter of finesse and focus, not power and force.
How do you play an overblow or overdraw? Good question. Hard answer.
First, you have to have your reed gaps set close to the reed plates. It also helps to have harps with good compression, that is good "air tightness". If your harp is set up improperly, you simply won't be able to get an overbend, and if its not set up just right it will be much more difficult to get the overbend. You can't count on being able to overblow any harp right out of the box, but just about any harp that comes in all 12 keys can be adjusted to achieve overbends.
Overbends are advanced techniques that require a lot of control of your mouth, throat, and tongue, as well as your playing pressure. An overblow is NOT a result of extra hard blowing or anything of the sort. It is a technique of finesse, where your tongue and throat have to be set in exactly the right position and your activation pressure has to be just right. Don't even think about trying for overbends unless you can easily do and control all the normal draw and blow bends.
The holes in which overbends add new notes not available through normal play or normal bends are:
You can't really sneak up on an overblow--you've got to snap it into place right from the beginning. Try imitating the sound of a jet engine. Form an O shape with your mouth and hump your tongue up at the back to constrict the back of the mouth at the soft palette, and push the air through the constriction with your diaphragm. Don't blow too hard! It's not necessary to force it. You have to finesse it! Try doing a draw bend in the hole first, then go right to the overblow. You don't have to change your mouth position much from the draw bend to the overblow.
Not getting it? Try this: hiss like an angry cat, with your mouth open and the hiss coming from your throat. Now use that to start a focused blow bend on hole 6. If you get the pressure just right, the overblow should pop out there for you. Don't forget or discount the advice to adjust the reed gaps close to the plate! Getting the gaps right is part of the overblow experience.
Once you get an overblow, practice holding it as long as possible. Relax everything you can without losing the overblow note. You'll find that you can get an overblow with much less effort than you think if you have your tongue and mouth in exactly the right position and shape.
Despite the tight tolerances required to get overblows, with enough practice you can actually bend the pitch of an overbend, and use vibrato on overbends. This requires a very well set up harmonica, and very good control that only comes from a lot of practice.
Another way to go is to practice on a Discrete Comb, which isolates the draw reed in its own chamber and accomplishes the same thing as blocking the blow reed with your finger or some tape, making overblows much easier to get. Even if you don't use the Discrete Comb as your standard instrument, it is quite useful as a practice device.
Overbends take a lot of practice.. probably more than almost any other diatonic playing technique. Then, once you can do the overbends it takes a lot more practice to use these extra notes to play the diatonic harp chromatically. Overbends open up all the positions to all modes of play, and bring jazz to the domain of the short harp. But it's not easy.
Again, make sure you adjust your reed gaps as close to the reed plate as you can get them without causing the reed to choke. Adjust the gaps close for both reeds in the hole. In other words, to set hole 6 for overblows, gap both the draw reed and the blow reed close to their reed plate. An improperly gapped harp is extremely difficult if not impossible to overbend.
The use of overblows has spread rapidly in recent years, primarily due to Howard Levy and his students. They provided the "missing link", technique wise, that bring every note and full chromatic capabilities to the little diatonic harp, though proper intonation and timbre are troublesome. Not to be overlooked is the contribution of Joe Filisko to the overblow revolution. Joe is a superb craftsman who has developed the customization techniques to build the superior instruments that have made overblows much more stable and accessible to the ordinary player. He, Richard Sleigh, and Jimmy Gordon build Filisko-method customized diatonic harps, based on the Hohner Marine Band and Special 20 harmonicas, that overblow with extreme ease. Unfortunately, Joe and Richard are so busy they aren't taking new customers. At this time, Jimmy Gordon is still taking order, but be prepared to wait a good while, currently 4-5 months. They aren't cheap, but they are great. Once you have one you can have it repaired for about the cost of a normal harmonica. http://www.customharmonicas.com/
Learning how to overbend is one of those techniques that can have positive impact on all your playing, including non-overbent notes. You gain control over your mouth, throat, and tongue position, become more in touch with your air stream and playing pressure, learn how improve your harp set-up, and enhance your vocal tract resonance and overall focus. It's well worth the effort required to learn how to incorporate overbends in your playing.