
Title link takes you to a new addition to the Rhodes Chroma site - the original documentation from Fender on the Rhodes Chroma Poly-aftertouch. BTW, if you haven't checked out the Rhodes Chroma site, do it. It's probably the most comprehensive and active sites dedicated to a given synth.
"The Pressure Sensor is a fully programmable, polyphonic, after-touch, pressure sensor. With it, the performer can effect one or more notes, independently of each other, in a number of programmable ways. The degree of the effect is controlled by the amount of finger pressure applied to one or more keys after they have been struck. The Pressure Sensor can be used to bend the pitch, add vibrato, control sweep speeds, accelerate attack times, accelerate decay times, modulate wave shape width, add or subtract brilliance, and/or control loudness. Any one effect or a combination of several effects may be used in a program. The Pressure Sensor is truly polyphonic; only the note sounding from each pressed key will be affected. Also, the effect can be latched if the key or keys are released rapidly. For example, if a note is bent sharp and the sustain pedal is depressed and the key is quickly release; the sustaining note will stay at the bent pitch."
Unfortunately most Chromas do not have polyphonic aftertouch. Even without, it is one of the most expressive synthesisers ever made.