Aftertouch
Related to velocity and equally useful as a modulation source is the aftertouch MIDI data type. While velocity reflects the speed with which a key on a MIDI keyboard is pressed, aftertouch is generated by applying subsequent pressure to that key while it’s held down. Unlike velocity, which is a ‘one-shot’ affair sending out a single data value then requiring a new key press to send another, aftertouch delivers a continuous (although it’s not a Continuous Controller in the literal ‘MIDI spec’ sense) stream of values between 0 and 127, as the pressure is increased and decreased, until the key is released. This makes it much more expressive than velocity when it comes to modulating filter cutoff, LFO depth, etc, as smooth ongoing progressions can be performed, rather than the jumps from one non-contiguous value to another that velocity triggers.
For most synths and MIDI keyboards, aftertouch is a channel-wide parameter, applied equally to all or certain groups of notes. There are quite a few applications, plugins and keyboards out there that support polyphonic aftertouch, though, including Bitwig Studio, Logic Pro, Kontakt, Omnisphere, the KMI QuNexus and many more. With a polyphonic aftertouch-compatible keyboard, each key sends out its own independent aftertouch stream, all of which can be harnessed by any polyphonic aftertouch-compatible receiving software for modulation purposes.
Velocity and aftertouch each have their own uses, but the two are really intended to be deployed together, with velocity setting the starting point of a modulation that’s then ‘wobbled’ using aftertouch.