Yoku Wakaru Mahouka! - Saiseitte Nani?

This ONA about The Irregular at Magic High School is a chibi animation about Tatsuya's powers. It follows the same format as the other 6 ONA episodes listed as this show's prequel.

This ONA has really poor animation because it's just chibi characters speaking. The main reason for watching this would be to gain a better understanding of Tatsuya's powers which weren't explained too clearly in the anime. You could also watch the ONA if you really like Tatsuya and Miyuki as characters and want to see them interact for an extra 3 minutes.

Overall this ONA is not worth watching unless you want to ... better understand Tatsuya's powers or like seeing him and Miyuki.