Shift is a party drug for the surface of reality, the para-dimensional super-natural effect of magic on the physical world. It makes you more yourself, the person or not-person that you would most want to be. Compared to permanent transformations, shifting is an easy technique with very few side-effects. It's perfect for the amateur witch who wants to play around with a new body for a while— but like all drugs, it can be dangerous in large quantities. Some say that it has a psychoactive effect, altering the person's ideal form until they lose track of who they thought themselves to be. Others believe that it is addictive in nature, doping the pleasure centers of the brain with irresponsible chemicals like "self-actualization" and "gender-therian euphoria."
Humans who spontaneously gain access to magic are prone to shifting into the shape that their magic suits best. Without proper training, this acclimation can become a permanent change; to many magical benefactors, this would be the entire point.