I’m quite sure I know how this works, I’ll try to explain but I have to begin a little with Adam and Eve…
What do you need to produce a note on a melodica?
1. You need a reed plate with a slit.
2. You need a reed that is just minimally smaller than this slit and is nailed (screwed, glued, whatever) above that slit.
3. You need air.
Now, what happens physically when a note is played?
The airstream hits the reed on the side where it’s nailed on the plate and makes it swing in a constant frequency depending on the length, thickness, weight, material of the reed. This movement makes the air on the other side of the reed swing in exactly the same frequency, and as we all know, this causes a tone. That’s all, meaning what is wrapped around the plate (an accordion, a harmonica, a melodica…) is of no importance for the production of the tone itself.
So, what happens when we want to bend a note?
We try to influence the frequency of the reed in such a way that it swings slower, i.e. the note we play becomes lower. There is JUST ONE WAY to reach this goal, it’s to accelerate the air stream. With this you bring the reed out of its natural position by bending it towards the slit. Now the reed swings its normal way in one direction, but in the other direction it swings a longer way because of the material that wants to force its way back into the natural position (I’m sorry I can’t explain better, there is a name for that physical power but I don’t know it). Fact is, now the reed swings a longer way i.e. it swings slower i.e. the note sounds deeper.
Everything we talked about, may it be increasing pressure when blowing, moving the key as little as possible, tightening the air hole by covering it a little with your lips, tongue etc. – all these things only serve to accelerate the air stream. The tinier the hole of the blow tube, the faster the air stream – the tinier the vent hole, the faster the air stream – he bigger the blowing pressure, the faster the air stream. All of those techniques may lead to a bending of the reed and, as a result of that, to a bending of the note.