Let me correct, with more precise language, the following paragraph from my post on the melodica apron. (I was tired last night and writing fast.)
“A good seal enables you achieve several conditions: a build-up of back pressure downstream of the reeds and valves before you play a note; a buildup and conscience variation of back pressure within the melodica chambers while playing a note; a muffled sound (which you can then contrast with the normal bright sound); and other modulations.”
Sealing the melodica’s sound output path (sealing the back of the melodica it against your body) does not build up pressure downstream of the valve and reed. But it does seal the sound output channel from atmospheric pressure. So when build pressure in the reed chamber and release it by pressing a key, the air flows more slowly pass the reed. The pressure begins to build downstream from the valve and reed. This is a condition not achieved in normal playing, where the pressurized air in the reed chamber flows through the reed without restriction (except for the resistance of atmospheric pressure).
This pressurizing of the downstream side of the reed is what harmonica players do when they cup their instrument to seal it off from atmospheric pressure. The sound is muted when they do this as well. When the harmonica player releases the cup while playing a note, the air rushes out past the reed at a faster velocity and into a rapidly changing downstream pressure. The tone brightens up.
Anyway, by manipulating your breath and the melodica in various ways (sealed, slightly sealed, open, rapidly alternating between sealed and unsealed, and many other gestures), you can obtain a significant number modulations including: wah wah effect, timbre modulation, volume modulation, Leslie effect, sounding of sympathetic notes, harmonic distortions, and more.
At least that is the way I see it.