This particular episode was looking at American couples and relationships, and it was analysing couples that are having problems and facing divorce. They did a very focused test which measured heart rate, stress, breathing etc. when in a forced stressful situation, between the couple. Then they would observe them more generally in a specially created flat doing normal day-to-day stuff for about 36 hours. Both these situations were then subjected to very detailed statistical analysis on many levels. The couples were given some therapy anyway, from what I could tell, and then they were further tested at later times up to several years later. The idea being that we can hopefully determine what makes relationships work.
One thing that seemed apparent from the programme is something I have noticed in past relationships, in that you tend to either be in a diverging or converging mode. In the focused analysis they measure what they called "repairs". They think this is vital to keeping a stable relationship, in that arguments and stress are part of everyday life, but the ability for each person to repair in these situations is vital. When one or both don't repair, things go wrong.