A MIDI keyboard responds in two ways to the player's touch on the keys. The first is velocity sensitivity. The faster the player presses a key down, the higher the velocity data produced. To the player, it feels like he or she is playing the keys harder, not faster, but the result is the same.
The velocity data is most commonly used to increase the level of the sound with harder playing, and also to increase the brightness of the sound, as would happen in an acoustic instrument.
Once a key is played and held down, there is also 'aftertouch', more properly known as 'pressure'. The player can exert more pressure on the key while it is down; this data is used to control typically a vibrato effect, level or perhaps brightness - it depends on how the tone generator is programmed.
But aftertouch or pressure works in two ways. One is 'channel pressure', where the keyboard senses the pressures of all of the keys held down and generates one set of data to cover them all. Usually this is the pressure of the hardest-pressed key.
So with channel pressure, you couldn't press one key hard and another gently and get different results for each key. Channel pressure covers the whole of the keyboard (or the whole of one MIDI channel).
Polyphonic pressure or aftertouch however works on individual keys. So all the subtleties of your playing are maintained, and if this data is used to control vibrato, then each note held down can have a different amount of vibrato.
The problem with polyphonic pressure is that it generates a tremendous amount of data. Even channel pressure generates a lot because it constantly changes. A pair of note-on and note-off messages comprises a mere six bytes, but channel pressure varies every time the player changes pressure on the keys. Polyphonic pressure multiplies this data by up to ten (ten fingers!).
The standard MIDI system can only transmit about 30 kilobits of data per second, and polyphonic pressure can easily push against this limit. Try adding more data, such as additional channels, and the whole of the MIDI data stream will slow down.
Now that MIDI via USB is common, and also mLAN is starting to make an impact, there is no practical restriction on the amount of data MIDI can use. So we should see more keyboards becoming available that can generate polyphonic pressure. Increased subtlety of music making will be the result.
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