- SPOILER - Show Spoiler
What if there is more to his spell than just a bunch of words to follow along with? What if there is also a compulsion to keep you on the same song and continue until the song ends.
Not specifying which song. Just until the song ends.
Now if someone didn't pick a song what if they are forced to start singing the first song they know in full that comes to mind. And what if they get the song in the spoiler to "pop in there" as Ray Stantz can tell just happens.
- SPOILER - Show Spoiler
Can't you just see some Muggle born students having their eyes go wide and panicky knowing about the nature of the song being sang by whomever that happens to?