If I recall, this is mostly like isometrics. You isolate muscle groups and do very intense but slow and deep contractions and releases that really make a difference over time (and not a lot of time, either).
I first did Callanetics in the mid-1980s and reduced my measurements by quite a bit, very quickly (but this is when I was in high school and generally fit and at a healthy weight, compared to the decades since then).
I tried Callanetics again in the late-1990s (when I weighed somewhere in the 185 to 195 range) and found it to be less effective.
I assume that just means the results are OBVIOUS on a smaller person who is "tightening" and "pulling in" quickly, vs. on someone who has more serious weight to lose before the results show.
Now that I've done Pilates quite a bit, I think there is some crossover there as well, but only in some of the exercises.
Most of all, I remember doing MANY more reps of each exercise than I had done on any other program, but the movements are so very small and fast that it's actually DECEPTIVELY EASY to do.
I loved Callanetics each time I did it. (And some of the movements, you can even do from bed--so it's a great choice for someone JUST getting started with exercise and muscle work.)
I believe I bought my VHS tapes used in a bookstore back in the '80s and still have them--but have seen DVDs and books on eBay and such since then.
That Callan lady is a force of nature, for sure! Does anyone know if she's still alive? I recall her being quite old in those '80s VHS tapes (of course, I was a teenager, so perhaps I thought she was "old" because she was over 50 or something. Ah... how times change).
Have fun!