As far as I know, the game doesn't tell you that bit of information. There may be NPCs that tell you but the game doesn't feel it's important enough to force you to run into their conversation. Breaking Magus's spell on the mountain is most likely what the game is implying but if Frog can in fact hold the power to break Magus' spells at a flick of the Masamune, then he has the power to defeat Magus quite easily. Of course, during battles, Frog doesn't possess the same tech or epic-beam-from-the-sky ability to easily thwart his enemies/Magus. In this comic, I'm merely exaggerating that weird logic.