It primarily affects the touch - which does affect the tone, but in ways that are too complicated and too nuanced to explain in a video.

check my reply to Wenneguen below

Luke Watts , the answer to your question is no. On a synthesizer, aftertouch comes from an additional pressure sensor. There is a velocity reading taken when the key is pressed. Once the key is down synthesizers with aftertouch has a pressure sensor. The pressure applied to the keybed can be used to modulate different synth parameters such as lfo pitch modulation, filter cutoff and similar effects.

@@jamesharrison342 Yeah I've since gotten a midi keyboard with after touch and it all makes more sense now. Shouldn't be called the same thing though. Causes alot of confusion for beginners

I’ve viewed like half a dozen of videos about pianos having exactly the same question. Now I finally know the answer.

For those interested, the polyphonic aftertouch in synths is a bit like aftertouch in a clavichord, where pressing the key after playing the note changes the pitch of the sound. So if you are looking for the source of the term aftertouch in synths, you have to look for an instrument few centuries older than the piano.

Well...yes, but not necessarily as a separate regulation. It's kind of complicated...it makes a BIG difference to the player so too much or too little aftertouch really affects the way the player manipulates the key and therefore the tone. But just regulating aftertouch a little does not necessarily have an impact.

So it means that AFTERTOUCH influences the sound/tone INDIRECTLY by affecting the playing of the pianist via mechanical feedback?

As I said in the video: aftertouch is the distance the key travels after letoff.

The term as used in piano technology is not related to the function of a clavichord action

It's just a regulation problem. You can raise the capstan if it's a grand or move the hamer rest rail more inwards if it's an upright.

Ha! I wish. More like truck and commuter rail traffic

Eric Johnson hehe sounds like that on my bose headphone

There are many variables. For example if you have excessive aftertouch because the letoff is set too low(wide) then you are loosing power. If you have too little aftertouch because of shallow keydip, you loose power because the key doesn't go completely through its travel. Too much aftertouch because of too much keydip doesn't affect tone very much but makes the piano much more difficult to play, affecting tone.