Hi Sam, I will try the aftertouch issue, although you may have to wait for someone more knowledgeable to correct my answer.

The short answer is that aftertouch does whatever it is assigned to do, and that can vary from voice to voice. On the PSR-8000 the aftertouch implementation is very disappointing. Only a few voices use it. Try the sweet tenor (sax) and the pan flute, and you will find that aftertouch controls vibrato. I think that Yamaha took a very conservative approach to aftertouch, because many of their "regular" customers are still have trouble with the idea that the sound varies according to how hard you strike the keys.

In principle, aftertouch can be used to control many different parametes. It could control pitch, volume, filter cutoff, etc. I think that aftertouch is one of the most important features of a keyboard because it is a continuous controller (like the pitch bend wheel or the mddulation wheel) but right at your fingertips. The three continuous controllers differ in their defaults--the pitch bend wheel is center sprung, the mod wheel is unsprung, and aftertouch is sprung at zero. Therefore, each has functions they are better suited to. However, in principle, as opposed to how they are implemented on the PSR-8000 each could be assigned to any function. I have demoes a PSR9000 and found the aftertouch implementation to be more useful than on the PSR8000.

I demoed a Roland XV-5080 sound module yesterday using a Roland XP-60 as the controller. The aftertouch implementation on the stock sounds was quite inspiring. You could use aftertouch to pitch bend up koto strings, for example. For me, originally a guitar player, using aftertouch for pitch bend up is very natural.

Althouch I question how useful you will find this information, I would be remiss if I did not point out that there are two types of aftertouch, channel (monophonic) aftertouch and key (polyphonic) aftertouch. The PSR8000 has only channel aftertouch. Low end arrangers have no aftertouch, most high-end arrangers have channel aftertouch. The only arranger keyboards that I know of that had key aftertouch as well as channel aftertouch are the General Music S2 and S3.

Polyphonic aftertouch is the most expressive of all the controllers in the midi specification. It allows module the different notes of a chord by different amounts. For example, you can bend the pitch of two concurrent notes by different amounts, as you might on a guitar.

As a feature, key aftertouch is dying out, largely because it is expensive to implement and because synthesists tend to be trained on pianos, which do not use continuous controllers. I hoping their is a resurgence of interest in key aftertouch once players start treating the synth as an instrument in its own right, rather than an emulator of acoustic instruments and vintage synths.

If they are going to emulate a vintage synth, they should emulate the Prophet T-8 which had key aftertouch. Another very rare synth with key afterouch was the DX1. Ensoniq used key aftertouch on its early synth (the SQ1) and some samplers (after the Mirage, but I forget the model).

Key aftertouch is expensive mainly in the keyboard, which must have a sensor for each key. I am not sure there are any keyboards in production that use key aftertouch. Many sound modules in production support key aftertouch. I am sure Kurzweils still do, the high end Rolands do. The original Emu Proteus line supported key aftertouch, but this feature has been dropped from the Proteus 2000 line (otherwise I would buy the new Planet Earth module).

I trust this is more than you wanted to know.