What is core shift?

I went to Washington DC for 5 days last spring-early summer. Its SUPER NICE down there. If you could get in working with the US Park Police I say go for it. It might be boring thou, many officers I seen were just standing around keeping an lookout, they don't speak to the tourist unless the tourist is doing something they should not. Like I locked up my bicycle near the Capital building and nobody says anything to me but as soon as I walked away from the bike I got yelled at that I can not leave my bike unattended--guess it could have been a bomb, sure you know how that game goes.