The definitions were written to help instructors and other serious casters communicate precisely with each other. They are not necessarily intended as teaching terms, and the document says so: it respects a colleague’s right to use terminology suited to their own students. A phrase such as a concave fly leg that crosses the rod leg is exact and checkable between instructors; to a beginner standing on grass it is a riddle.

None of that makes the definition wrong, or the mechanics optional. The tip path, the stop, the pause and the arc matched to the bend still have to be taught correctly. What changes is the language and the order in which it arrives. Say that the top of the loop dipped under the bottom before you say concave fly leg; say stop the rod and let the loop go before you say deceleration resulting in loop formation.

The definitions also settle arguments between instructors, which is much of their job. Use them to check what you are teaching, not as a script to recite. A student who repeats a definition back to you has learned a sentence, not a cast.

Source

From the introduction to the FFI Fly Casting Definitions (August 2025), on communication and teaching terms; the full text is on the site at FFI Fly Casting Definitions.