The paper describes mathematically how that happens. First comes the process of unlearning the old paradigm before learning the complex new one. If the errors are big enough, I have to reconsider the entire model. The next move the subject makes likely is worse than the one before until the subject starts realigning the primitives correctly. Because the changes in the reality were done deliberately, not randomly, Shadmehr has been able to predict with considerable accuracy what the subsequent moves will be, how the primitives are changed.
Experiments show that the theoretical limits are closely observed in people's learning abilities.
In other words, humans have a limited number of primitives to deploy when learning skills.