Hi jewels5. I don't have knee issues so I cannot address your first question. But I can describe the workout to you somewhat, so hopefully that will help you (and you should definitely watch the video clip on Shop Cathe).
Rhythmic Step is definitely a more heavily choreographed workout. After the warm-up, there are 3 separate step sections and then a 9 min "challenge" where you put each of the 3 step sections together. If I remember correctly, the first step section has a move where you do something like a mambo on the step + cha-cha on the floor + pivot on the floor + cha-cha back the the step. (Lots of twisting on that move!) There is also an optional move where you jump/pivot around the step (but you can just follow the original version of the move she shows you, which requires no jumping/pivoting). The second section has triples around the step and another mambo + cha-cha move. I don't remember any twists or turns in the final step section. So, yes, there are quite a few times where you will be twisting/turning/pivoting. I've never tried to modify the workout to take those out but it seems like -- for the most part -- you could very easily do so.
There are also moves throughout the Rhythmic Step that involve jumping, but Cathe usually introduces the moves with a low impact modifier. Plus, it sounds like you are already used to modifying moves to low impact, so this aspect of the workout shouldn't be a problem for you.
I love this DVD because it also includes the original Imax and Maximum Intensity Cardio. While Rhythmic Step is fairly easy to modify for low impact, these 2 other workouts really are not, so I don't know how often you would want to use them --if ever-- given that you seem to prefer low impact. That might be something to consider as you make your decision.
