Aftertouch (or Pressure) is a MIDI message which represents the pressure you apply to the keybed after a key has been struck and is being held down. The Seaboard uses this message to transmit Press.
The pressure you apply to each keywave is measured continuously – not just when you hold against the keyed – which means that detailed expressions like swells and fades are possible right from the first Strike of the note to the final Lift.
When you strike a key on the piano, the speed at which you hit the key determines how hard the hammer swings into the strings and so, how loud the note sounds. On the Seaboard this is called Strike and is transmitted using MIDI's velocity message.
If you hold the key down and press harder on a piano, nothing will happen; the hammer has already swung and is resting.
Some keyboards can measure how much pressure you apply to the keybed after the start of the note (Aftertouch). Aftertouch can be used for extra modulation during the note's sustain for example to increase volume or a filter so that pressing harder makes the instrument sound louder or brighter.
Most keyboards apply aftertouch to all notes equally as a single value, but the Seaboard allows you to apply aftertouch to each note individually. The Seaboard also applies aftertouch as a continuous value so that instead of starting from 0 after the initial attack of each note, it is with the Strike and at a value according to the Strike value. This means that it is also possible to fade in from silence with very gentle keywave presses.