Isochronic modulation frequency and modulation depth

I am a novice, downloaded Audacity yesterday, today was my first day working with the program, definitely lack the engineering skills many of you seem to possess. My current main interest is Isochronic tones, my question references isomod plug-in - may I please impose that someone define for me the terms initial/final modulation frequency and initial/final modulation depth? Also, why are the initial/final modulation frequencies relegated to 1-20 Hz - does this infer that if the frequency of the tone is 100 hz it begins and ends with 20 Hz? Thank you for any feedback.