Tailing loops are a study in and of themselves.
For the most part, stopping the rod at or above the path that the line is coming forward will result in a tailing loop. Stopping the rod under the path of the line coming forward generally will not.
A concave path means the line coming forward took a dip and is going to dip as it unravels. A very early hit causes the rod tip to bend deep too early and it recoils upward on the latter half of the cast, thus the line coming forward is aimed lower than the stopping force of the rod and tails. A slow creep to a hard hit late has a low-speed low-flying line coming forward with a stop force much higher in altitude, thus tails. The root cause appears to be that the stopping force occurs above the path of the line coming forward.
Why Lefty himself says that the only cause of a tailing loop is when the rod stops at or above the path of the line before the stop was done.
I am beginning to believe very strongly that (a) the straightest and longest possible path that the line can acclerate over combined with (b) the most sudden possible stop underneath and to the inside of that line path = the tightest loop and most efficient cast possible. This is consistent with what many 'experts' have said, I should look up the actual articles and post the references here for anyone interested.
I have found these principles to be of very high value to think about for personal and instructional purposes.