What is polyphonic after touch?

Polyphonic aftertouch is a feature of MIDI keyboards and synthesizers that allows a player to vary a parameter on each note independently, based on the pressure on the key, after the note is struck. It can be very expressive, but it takes a lot of practice to develop the finger dexterity to use it effectively.

Polyphonic aftertouch is a very rare feature, because it is expensive to implement and few people can use it well. Most keyboards with aftertouch capability only provide channel aftertouch, with is based on the average pressure of all the keys that are pressed.

Even if you do not have a MIDI controller with polyphonic aftertouch, some MIDI sequencer/editors can be used to add it to the performance data.