I am probably wrong but I can only speak from my experience of trying so many ADHD medications. Fidgeting is more of a peripheral nervous system issue. Cognition is a central nervous system issue. ADHD-PH = Primarily hyperactive goes for your restless and fidgety types. ADHD-PI = Primarily inattentive goes for your otherwise calm, but less receptive to give attention to information and tasks. The best analogy I could ever think of is a baseball glove. Part of my mind has hands and another part has a baseball glove. With ADHD, the glove is ill-fitting and I cannot catch the data coming through all the time. My hands are able to synchronize themselves and actually crave for things to fit but there's a disparity, as it were. In practical terms I would get lost while trying to pay attention in class and while reading dense text. When I went on Concerta I felt like the baseball glove was fitting better. When new information came in my mind caught it snugly, there was a feeling of "I got it!" while I tucked it away with the other hand and snapped back to catch more information with the other.