Its just like it sounds. US CBP diverted all traffic exiting the US to the inspection booth and asked questions about what we were doing in the States, etc. They scanned docs. When we pulled up to the booth on the Canadian side, the CBSA officer couldn't have been bothered much, asked us where we lived, what we had and then waved us through. He didn't scan anything. This has happened several times. Its true that most times, US CBP does not do exit inspections, but occasionally, when they are bored, or there is an Amber alert or a fugitive look out or something, they do.