6 overblow and 7 blow simultaneously
This effect provides a major second interval which sounds rough and edgy. It’s a nice expression effect. You can do a vibrato on it, you can combine it with 6/7 draw (which is the major 2nd interval also) if you play blues in second position for example. To apply this effect, do an overblow and widen up your mouth slightly to the right, keeping the overblow.
Bend on 2nd hole and straight 1st hole.
This is the technique you can find Howard Levy doing occasionally. You’ll get additional intervals by applying this technique. On C harmonica you’ll get D & F# notes (major 3rd) if you do a half-tone bend and D & F notes (minor 3rd) if you play a whole-tone bend. The way you apply it is similar to the previous technique, you keep your tongue in front of a hole and widen up your mouth slightly (!) to the left side. In 2nd position you can support the V chord by playing this interval.
Advanced tongue blocking.
This is a technique that includes varying the amount of holes you block by moving your tongue. Again, Howard Levy is a perfect example, check him out playing Bach’s “Jesu, Joy of Man’s Desiring”, you can find different kinds of intervals here. This technique may be challenging because you use more air playing the interval than playing single notes, so you’ll need to take breaths more frequently. Focus first on switching certain intervals, like 1xx4 and 1xxx5, then 1xxx5 and 1xxxx6 slowly and consciously, with a metronome set at around 60 bpm. Play one measure in 4/4 on exhale switching intervals every quarter note, then on inhale. I recommend using harmonicas with short distances between holes, such as Marine Bands. The bigger the interval, the deeper the harmonica will be in your mouth.
Using your voice while playing.
This is as simple as using your vocal cords while breathing out into the harmonica, but it requires good intonation, so you’d probably need to practice vocals separately. You can play a fifth or an octave on the harmonica and sing while you play.
Mouth breathing only.
This effect can allow you effortlessly to play really fast notes in the high register. As short high notes on the harmonica don’t require much air, you can breathe just with your mouth, which will make a faster attack than diaphragm breathing. In the video above, listen how it sounds with a chord, and how it sounds with high notes. You also can combine it with diaphragm breathing, and you can do dissonant sounds in the low register by applying this effect.