That piece of information comes from an article on Breaking Muscle.com from Doug Dupont.
Have you ever thought about training partners that way before? As I read this article I expected Doug to tell me that the third group was competing against one another. Instead, its a team approach.
The next question – is the motivating factor the fear of letting your teammate down or the fear of losing? I believe the answer is the later.
I used to teach college classes. Part of the course requirements were to stand up in front of the class and give a five minute presentation. In the beginning, it was every man (or woman) for himself. When assigned as an individual project the results were not great. There were always a few students who did not put forth the effort. Later, the format was changed to a group presentation, The results were outstanding. I attribute the improvement to the peer pressure element. When other people are counting on you it makes a difference. The speeches were much better.
In the classroom, in the gym and in many other areas of life, the takeaway is that people will try harder to support other people than they will to rise above it.