Ring modulation is created in HighC exactly like frequency modulation synthesis: by moving modulation curves inside the audible range instead of leaving them in the low (inaudible) portions of the score. The difference is that the modulators must modulate in amplitude instead of frequency. In this sense, all the parameters of ring modulation are the same as low frequency amplitude modulation.
Ring modulation has some common properties with frequency modulation: it consists in twisting the spectrum of audible sound by introducing other frequencies within the audible range, which results in richer sounds. Because, like FM modulation, the modulator signal can be varied in amplitude and frequency, this allows to modulate the timbre of a sound along its duration.
However, ring modulation, tends to be less powerful, and it is a tad more difficult to master. The main reason is that, while sounds modulated in frequency tend to keep their main pitch and just enrich their spectrum, ring modulation twists the central frequency in the sounds, and produces a compound sound whose main pitch is different from the original modulated sound. This means that the pitch you see on a score for the modulated sound has little to do with the pitch your hear.
More precisely, a sound of frequency f1 modulated by a sound of frequency f2, it will produce two sounds at the frequencies f1+f2 and f1-f2. Unless f1 and f2 have a lot of common multiples, the result if a chord of 2 sounds (plus spectrum distortion) which are not harmonically related. So, most of the time, you'll want to keep the ring modulator at an harmonic (octave, 5th, 4th...) interval of the modulated sound.
Note also that ring modulators can be combined and cascaded exactly like frequency modulators. The main difference is that, because ring modulation is a simple multiplication of the signals, it is transitive. Hence, cascading and combining ring modulators results in pretty much the same effect (modulo the aliasing and phase transition effects that may appear here and there).
Note: a future version may improve the graphical representation of a ring modulated sound, by displaying the resulting main pitches as "shadow sounds" above the modulated sounds. This should help controlling the output pitch, and therefore, make ring modulation much more useful than it is now.