What do these phrases actually mean, and how would you explain to a foreigner how and when to use them?
It is basically a way of indicating to the listener that you were speaking figuratively, and what you were saying is not to be taken literally to aid someone's understanding of what you have just said. For example if you were speaking to someone unfamiliar with the phrase you might say : "she wears the trousers in that relationship, so to speak." You don't dont literally mean she is the only one to wear trousers but that you were speaking figuratively about her being incharge.
