Ying and Yang, Up and Down, all are opposite and always used in the same sentence and street photography is no different when it comes to people.
So the objective or brief here is to look for something which contradicts the other, in the image below you will see the man asleep obviously enjoying his little sleep while the hassle and bustle of New York goes on around him during rush hour. While in the background the grumpy sad faces of commuters in the summer heat getting the trample on their mood.
Such a great moment to photograph and capture.
I love looking for this kind of thing and it really brings in a story all of its own.
Another way to look at the “opposite” edge is don’t ‘just’ focus on people add the environment too, start looking for things that offer reflective surfaces bouncing back the image your looking at for example.