“Today, children,” began the calm professional voice of the Transfiguration Professor, just as though nothing out of the ordinary had happened that week, “we shall learn how much effort it takes to sustain a Transfiguration, and why, at your age, you should not even try.”
“Um,” Harry said. “Actually I was thinking that once I know how, I could Transfigure the rock into a ring and wear it on my finger. If you could teach me how to sustain a Transfiguration—”
“It is good that you asked me first,” Professor McGonagall said, her face growing a bit stern. “If you lost control of the Transfiguration the reversal would cut off your finger and probably rip your hand in half. And at your age, even a ring is too large a target for you to sustain indefinitely without it being a serious drain on your magic. But I can have a ring forged for you with a setting for a jewel, a small jewel, in contact with your skin, and you can practice sustaining a safe subject, like a marshmallow. When you have kept it up successfully, even in your sleep, for a full month, I will allow you to Transfigure, ah, your father’s rock...” Professor McGonagall’s voice trailed off.
So, if it is a training method, it’s one McGonagall knows about and in fact specifically suggested.
What anomalous magical power?
So, if it is a training method, it’s one McGonagall knows about and in fact specifically suggested.