As you may know or not, MCSC stands for "Multi Character Star Cancel". Star canceling is a glitch performed by Rosalina. By spinning into the star, the game doesn't play the star cutscene. This is most likely because that the games stars weren't meant to be collected with such moves as spinning with Rosalina. So, the run is basically where you try to beat the game whilest star canceling and switching from your character to Rosalina for each star.