org/wiki/Do_Black_Patent_Leather_Shoes_Really_Reflect_Up%3F , the general (humorous) idea is that black patent leather shoes might show a reflection of a view up a girl's skirt. I think more context is needed in order to understand the purpose of the word "only".
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.