This phrase was brought to life for me when I was a camp counselor. Our director asked us to be aware of trash around camp an pick it up. He planted the phrase in my brain, if not you, then who? To this day as I walk down the street and see a piece of trash I think of this phrase.
Here in NYC this phrase is alive and well-sometimes for good and sometimes for not. It would be surprising to see people here in the city applying this phrase and actually picking up trash off the sidewalk. The way I see this phrase come to life for me the most though is on my commute. It has come to life for me in a whole new way. I have learned that if I don't assert myself someone else will. If I don't fill in the space on the train or the stairs someone else will. Along the same lines if I don't give up my seat for a child or older person who will.
In summary, step into the gap, make your way around people so you can make the train, give up your seat, and remember you're the WHO.