Black's final words had worked remarkably well. As students, they naturally attached great importance to their fifth-year wizarding examination results.
Good marks meant more pocket money, proud families, and something to boast about to their friends in the future. Even Slytherin's little snakes were no exception.
After all, at this stage, they were still only students.
Even after class ended, the young wizards were still turning their wrists on the way to the Castle, silently working out how to pronounce the incantation.
For them, it had been a wonderful dark magic defence against the arts lesson, but for Su Er, it had been rather unremarkable. He had been forced to learn the Shield Charm as early as the previous school year, when he fought Snape.
Yes, he had taught himself. Snape certainly would not teach Su Er how to defend against his attacks. That would get in the way of beating Su Er up, wouldn't it?
Back then, to keep himself from getting beaten too badly, Su Er had spent a considerable amount of time in the Library.
A purely defensive spell that caused no harm, such as the Shield Charm, was naturally covered in the Library's books.
However, he had not been idle either. Su Er had been quietly considering another way to use the Shield Charm.
He had noticed that the shields Black formed while blocking Lupin's spells had varied in size.
It seemed they could be controlled quite flexibly according to a wizard's needs.
Thinking back to the magical theory Professor Flitwick had once explained, he seemed to be able to use this spell more efficiently, forming a much smaller shield when he could not dodge a spell. That would save a great deal of magic.
However, that required a very deep understanding of the Shield Charm.