This video gives a good, brief explanation of Relational Frame Theory (RFT).
FIML practice can be understood in terms of RFT. What FIML practice does is give partners immediate access to their neurotic “relational frames” of reference, their mistaken interpretations. When we see a few times with great clarity that our neurosis is based on a mistaken interpretation (a mistaken relational frame) of what our partner actually means or meant, we will be able to change our relational frame (correct our mistaken interpretation) without much trouble.
FIML works especially well for making this sort of change in relational frames because it deals with those frames the moment they arise, while they are still just starting to be accessed. FIML also works well in this respect because it is based on real data shared and agreed upon by partners who trust each other.
Here is another article on Relational Frame Theory.