I know there is a lot of misunderstanding about Key Pressure and Aftertouch (for the search engine here: After Touch too).
And you will find a lot of information mixing things together - not only here but everywhere in the Web.
Here is what the Virus (at least Ti and Ti2) is doing what:
- Key Pressure (CC129) is doing the modulation as expected: Pressing a key or multi keys will do the modulation as programmed in the Virus Matrix
- Aftertouch (CC131) manipulates the OSC3 setting of the Virus
Very often, if people talk about Aftertouch, when they mean Key Pressure - or Channel Pressure as it is usually called (see Matrix programming on the Virus)
So do not get frustrated (as somebody who asked me) if the result is not what is expected.