Cummins and Port asked participants to produce sentences over and over and examined when during the cycle a certain stress beat occurred. They set it up so that the beat was timed with a beep to occur throughout the cycle, but showed that people could actually only place the beat in 2 or 3 places in the beat reliably. The big picture result is that speech production is shaped, in part, by the underlying dynamics of production described in terms of the rhythms it is set up to produce.
The nice detail here comes from the theoretical set up and analysis that drives this study. Cummins and Port are directly inspired and guided by work in coordination dynamics. Agnes is interested in this work because she's looking at ways to investigate language and speech using the tools of dynamical systems and embodied cognition - remember, our big pitch is that language is special but not magical and we should be able to study it the way we study, say, rhythmic movement coordination.