Hi Victor -
A sudden speed-up causes shock. Shock in the line can deform and weaken the loop energy, and shock in the rod can cause early recoil in the rod and tailing loops or worse yet both can occur.
A gradual speed-up gives the line time to become straight, and moving in alignment which is most important before the final application of power. We think of these things as happening very fast but it takes time for line to transition from one direction 180 degrees in the other direction, and come straight before the stop.
Essentially, it does no good to stop the rod until the line is pulled straight, or has enough tension on it to come straight when the tip is stopped.
A well straightened line stopped in line flies as far or further with less effort than a hard cast line that was not straight before power was applied.
Starting gradually and speeding up prevents shock, gets the line in the essential straight line path, and saves the caster from using unnecessary power to acheive similar or superior results.
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