I just ran across this theory for the first time today. I was wondering if you experienced magi would share your thoughts on it's application and effectiveness as well as your interpretation of the theory.

Juan Tamariz is a great proponent of in transit actions.
A more easy to find source discussing this is the Card College series.
If I'm not wrong Kurtz talks about this, but I'm not sure he uses this term.
It has to do with one of the important rules of magic - keeping a natural flow. The basic idea is that a sleight or secret action is performed under the cover of a natural lesser action during a primary motivated action.
This way the lesser action, and the sleight it hides have less focus and the sleight is not noticed or the lesser action not remembered at later deconstruction.

Well put! I feel it is a VERY important, practical concept.

Vernon summed it up before this theory was given this name: Be natural. This theory has more to it, of course, but at its core, it means doing the "move" in the course of a natural action.