While a particular note is playing, pressure can be applied to it. Many electronic keyboards have pressure sensing circuitry that can detect with how much force a musician is holding down a key. The musician can then vary this pressure, even while he continues to hold down the key (and the note continues sounding). The Aftertouch message conveys the amount of pressure on a key at a given point. Since the musician can be continually varying his pressure, devices that generate Aftertouch typically send out many such messages while the musician is varying his pressure. Upon receiving Aftertouch, many devices typically use the message to vary a note's VCA and/or VCF envelope sustain level, or control LFO amount and/or rate being applied to the note's sound generation circuitry. But, it's up to the device how it chooses to respond to received Aftertouch (if at all). If the device is a MultiTimbral unit, then each one of its Parts may respond differently (or not at all) to Aftertouch. The Part affected by a particular Aftertouch message is the one assigned to the message's MIDI channel.
It is recommended that Aftertouch default to controlling the LFO amount (ie, a vibrato effect).
0xA0 to 0xAF where the low nibble is the MIDI channel.
Two data bytes follow the Status.
The first data is the note number. There are 128 possible notes on a MIDI device, numbered 0 to 127 (where Middle C is note number 60). This indicates to which note the pressure is being applied.
The second data byte is the pressure amount, a value from 0 to 127 (where 127 is the most pressure).
See the remarks under Channel Pressure.