Needs a Better Title. It's when a character of the show appears to sing briefly in the opening of a show. They sing just one line or part of a line (not to be confused with shows where a character sings the whole song). This can be done (usually in anime)to add emphasis to that portion of the song/opening.
Winry sang "yamete yo" in the first opening of Full Metal Alchemist Brotherhood.
Miss Shizuka sings "doko e iku" in the opening for High School of the Dead.
A lady sings "soge" in the opening for Super Robot Taisen OG - The Inspector.