Two caster actions can drive the rod tip into a concave path and produce a tailing loop: a casting arc too narrow for the actual bend in the rod, and misapplication of power — force applied abruptly or unevenly.

• A casting arc too narrow for the rod bend. The energy needed for the cast is applied over too short a distance, so the rod tip path goes concave. The fix is widening the arc to match the bend; reducing energy instead can leave too little rod and line speed to make the cast.
• Misapplication of power — abrupt or uneven force. The guide calls this one of the two most common causes. A jerky start, a late spike or an early burst can all dip the tip into the concave path; it need not be a slow start followed by a fast finish.

Do not diagnose every tail as simply starting too slowly. Check the line, the tip path, how force was applied and how much arc was available. For a straight tip path, accelerate the rod at about the same rate from the beginning of the stroke to the end. Creep can contribute: drag during the pause that narrows your arc leaves you applying power over too short a distance. Read the fly leg after every cast.

Source

From the CI Study Guide (2025) (PDF), Task 3 and the Five Essentials, plus the tailing loop task; terms from the FFI Fly Casting Definitions.