An important distinction is made in academic linguistics between competence and performance. This short entry in Wikipedia gives some idea of why—Competence versus performance.
FIML practice, unlike most academic linguistics, is centered on performance, not competence.
Through practice FIML partners will come to understand how their communicative performances affect them both. By extension, they will also learn how their performances in both speaking and listening affect other people.
FIML deals primarily with communication mistakes and their emotional impacts on both speakers and listeners.
FIML partners will quickly discover that they make many mistakes in speaking and listening and that when these mistakes are not corrected, many of them can have serious consequences affecting the well-being of both of them.
In a nutshell, FIML removes performance errors made between partners in real-time. By having a reliable method for removing misunderstandings as they occur, FIML partners will also find that they are able to pursue many more subjects with greater depth and intensity than they had been able to do before.
Many of the errors removed by FIML practice will more be of a “psychological” or emotional nature than a linguistic one. That said, it is important for partners to focus a good deal of their energy on specific linguistic mistakes because when both partners agree on exactly what was said, they will have excellent data that can be profitably analyzed.