Here is a fantastic article I found on Pat Missin's web site. Be sure to go to his site for other great articles.

Hand resonance

As a child, you may have learned to make owl noises by blowing over your cupped hands - not surprisingly, there are several websites devoted to this pastime (see the links at the foot of this page).

Technically speaking, your cupped hands are acting as what is known as a Helmholtz resonator. You can use this effect to help give increased volume and improved tone with your harmonica, a technique most closely associated with British classical harmonicist Douglas Tate. The basic idea is to alter the size of the chamber formed by your cupped hands so that its resonant frequency matches the note you are playing, or a harmonic of that note.

The basic technique is quite easy to learn. Start off by playing a single note (6 draw on a standard C diatonic is a good starting point, or 7 draw on a chromatic in C), with your hands forming a tight cup at the back of the instrument. Slowly and gradually open your hands a little, paying very close attention to the tone produced. As you continue to open them, you should notice that at a certain point, the note suddenly becomes louder and seems to take on a much fuller tone. As you continue opening your hands, the volume will drop and the fullness of the tone will decrease.

Here is an audio example of what I just described. I am keeping my breath pressure constant for the whole exercise. You should hear the point of fullness reached about 2 1/2 seconds into the sample:

AUDIO

Each note on your harmonica will require a slightly different hand shape - higher notes will require your hands to be more open - but it will only take a very small adjustment of your hands for each note. Once you find the "sweet spot" for a note, you can add a lovely vibrato-like effect with just the slightest movement of your hands - literally fractions of an inch. This effect is much more subtle and in my opinion, much more musical and much less corny than the typical "hand vibrato" used by many harmonica players:

AUDIO

This technique can even be used effectively whilst cupping some microphones, particularly with small condenser mikes. It may take a while to be able to use it to best effect, but it is well worth investing some practice time into it. Combining this technique with Overtone control can result in some extremely powerful tones.