When I ask for that demonstration Aitor, and if I do it myself, I try to ensure that arc is not removed and tails result. From my experience most tails from finishing the haul too soon occur on the back cast.
So I assume the "alleged cause" is a concave path of the tip of the rod, or is it something else? So the tip has to rise in a "brisk" way to create a tail? If it rises in a not so brisk way do we get a tail?