Overblows
A so-called overblow is a type of bend where the pitch that is created is higher in pitch than the natural note of either reed in the hole, rather than lowering the pitch as in an ordinary bend. This is because the overblow technique actually causes the normally-sounding reed to choke while you're playing so it doesn't sound, and causes the other reed in the chamber to sound as an opening note instead. Just as there are both draw bends and blow bends, there are overdraws as well as overblows, and they are collectively called overbends.
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 harmonica--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 harmonicas with good compression, that is good "air tightness". If your harmonica 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 harmonica right out of the box, but just about any harmonica 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:
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overblows on holes 1, 4, 5, and 6, and
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overdraws on holes 7, 9, and 10.
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Overblow technique varies considerably between holes 1 and 6, as do overdraw techniques between 7 and 10, so you pretty much need to work on each one separately. Most people find that the overblow on hole 6 is the easiest one to get first.
Overblows
In an overblow, your tongue is set in essentially the same position as for a blow bend on hole 8. The tongue and the throat constrict at the place where you make the hard "K" sound. The air pressure comes from deep within, from your diaphragm. Very tight control over the air stream and embouchure is required to start an overblow and to keep the overblow sounding after you get it. A slight waver in your control and the overblow will be lost.
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.
Overdraws
An overdraw feels like a strong draw bend with the jaw not dropped and the tongue set more in a blow bend position. Here again, the pressure used to get and sustain an overdraw is very critical with very little variation tolerated. Start it with a well articulated attack with the tongue, as if making a T sound, and play with a tight pull draw bend. Hole 7 is the most useful overdraw, though you may find it easier to get an overdraw from hole 8--but remember that hole 8 doesn't add a new note. Experiment with different pressures, because you have to get it just right to pop the overdraw note out there. Remember, you're pulling the note up in pitch, not down. You need to adjust your resonance to match the higher note.
More Tips
Still having problems? Not surprising. It is helpful to get the sound in your ear, and get the feel of what hitting an overbend is like. One way to do that is to remove the covers and practice overblows while blocking the corresponding blow reed. (Remember, it's the draw reed that sounds during an overblow, as an opening bend.) You can use a finger to block the reed, or you can even use a small piece of tape to do the job. It is much easier to get an overblow with the blow reed blocked, and you can practice this way to learn the sound you're going for. It's easier to get the note when you know what it is supposed to sound like--probably due to a subconscious tuning of the vocal tract to the resonant frequency of the note. It's kind of like whistling--you automatically make the adjustments in your mouth to get the note you're hearing in your mind's ear. Once you are used to getting overblows with the blow reed blocked you can transfer the feel of the technique and the sound of the note to normal play--with the covers on! I have heard of people who actually cut a large hole in the top cover so they can use their fingers to block the blow reeds and get overblows.
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 harmonica chromatically. Overbends open up all the positions to all modes of play, and bring jazz to the domain of the short harmonica. 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 harmonica 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 harmonica, 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 harmonicas, based on the Hohner Marine Band and Special 20 harmonicas, that overblow with extreme ease. There are many more customizers now, but I haven't tried their harmonicas.
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 harmonica 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.
6 Overblow and Blues
What makes the hole 6 overblow important for playing blues? In second position cross harp, the hole 6 overblow (6#) (See tab notation) is the flat 3rd of the tonic I chord, which is also the dominant 7th of the IV chord. It is part of the blues scale for the 2nd position key, e.g. G on a C harp, and you can't really play blues in the middle octave without it.
It's the same note as the hole 3 draw ½ step bend (3') (and the hole 10 full step blow bend (10'')). It's the "blue" note that gives blues its character! And it bends too, like the others. You can bend the 6 overblow up/down (6#') to the (non flat) 3rd on the I chord, so you can get the same flavor as when you play on the 3 draw bend.
It sits there right on top of the tonic in 6 blow (6>). Blowing between 6 and 6 overblow is like drawing between 2 and 3 bend. It gives a strong feel of going somewhere, or, when going back, of getting somewhere. Think of how often you use the 3 draw bend. That's how often you could use the 6 overblow when playing blues in the middle of the harp.
The hole 6 overblow makes the middle octave very effective for playing blues in 2nd position.
Being in the blues scale of the 2nd position key means the 6 overblow provides a playing path from the middle of the harp up to the high end. Without it there's a break in the flow between the 2 octaves (because you have to skip an important note in the blues scale).
So basically instead of mainly using holes 1-6 for playing blues, the 6 overblow opens up the rest of the harp!
Luckily, the hole 6 overblow is the easiest one to play.
This clip is an example of using the 6 overblow for blues just using holes 5-7.