It turned out that a partial Shift was not all that difficult. The first time I tried a partial Shift, I succeeded right away.
My hand transformed into a Hnoll's clawed hand, and the rest of my forearm followed easily enough.
Maintaining and restricting a partial Shift, on the other hand, was not as easy. First things first, partial Shifts were slower. It was even slower than my first few regular transformations had been. Good thing that was something I could work on. Theoretically.
Also theoretically, I should be able to maintain certain partial Shifts permanently. The Hnoll's clawed hand, for example, should be permanent – if I could maintain and restrict it. And that was where the first problem arose.
