... But what kind of concept is that? How can a key on a keyboard transmit a note-off velocity?...
Just as striking a key with more speed generates a note-on velocity, releasing the key more quickly or slowly can produce a release velocity value from 0-127. It's a useful parameter, but rare in keyboard controllers, and very rare as a modulation source in synths. Here, my later Novations, Alesis Ion, and pad controllers feature it. The early Novations, Oxygen8, classic hardware synths or drum machines, and any 'budget' controllers do not. My AKAI Max49 is right in the middle: True Note Off with a constant Release Velocity of 0 (and the MIDI section of the manual states that it doesn't feature Note Off Velocity).
A often-cited example of use would be a quill touching the string of a harpsichord after key release. In practice, it can be a very expressive device with programmable synth parameters. Or it can be as subtle as a slight guitar string detuning while it decays. You have to put in some real practice with the kind of technique - just like anything else. If you point release velocity to pitch, it can be a valuable practice tool. You can train your key release rate to produce a general range of pitch change.
When I release a key on my keyboard, it sends zero velocity, so the note can come to an end...
Is that a 0 velocity value after Note On, or a true Note Off message (with 0 velocity)? Two distinct MIDI Command Set parameters. Common MIDI practice uses a Note On message with 0 velocity as an alternate Note Off message.
Seems that my keyboard (M-Audio Keystation Pro 88) doesn't transmit Release Velocity, although in the Midi Implementation Chart says is does transmit...
So it does. Usually, you'll see Note On: v=1-127 or Note Off: 9nV=0 in the Chart to indicate the alternate MIDI Note Off message (Note On velocity of 0).
If you want to be sure, hook up a MIDI monitor (like "postcardware"
Bome's SendSX).
If a hard-struck key press & release on middle C shows:
... then you have MIDI Channel 2, Middle C Note On at maximum velocity of 127, followed by a Middle C Note On message at a minimum velocity of 0. If you see something like:
... then you have the same MIDI Channel 2, Middle C Note On at maximum velocity of 127, followed by a true Note Off for Middle C (Channel 2) at a variable release velocity [low-mid value shown here].