Gus was unsure what to do with the pair of magic panties he found. He had no idea of what kind of magic they could perform. He decided that he may as well work out what kind of magic they performed via trying them out. He was unsure of how to use their magic but assumed he had to put them on. So that is what he did.
As he slipped them on his body shivered. He suddenly felt closer to the ground and very light. His face burned as bones transformed to create a feminine head. His hair sprouted and his hips became wide and pronounced. His rear end softened and became round. Two mounds of flesh extended from his chest. Then finally his penis retracted to become a vagina.
Gus found out what kind of magic these panties could do and he liked it.