To get to the next couple's spot, the couples that are facing in simply walk forward, without turning. The couples that are facing out must flip over to get to the next couple's spot, turning halfway around to face the opposite wall in the process. Every couple either walks forward without turning or turns around exactly halfway (180 degrees).
Couples Circulate and Box Circulate are based on the same idea of a circulate path, with the same number of positions (4).
The difference is that in a Couples Circulate the units that are moving are couples, rather than individual dancers.