Creative Speculation

The forward head snap is explained by Bernd Karger from the textbook "Forensic Ballistics":

"When the bullet strikes the skull, the velocity abruptly slows, thereby transferring kinetic energy to the target. This primary transfer of energy causes the target to move minutely into the force and against the line of fire, quickly followed by movement with the force, and in the continued direction, of the moving bullet (Karger, 2008)"

Fiester's descriptions of back spatter explain exactly what is observed in the relevant frame of the Zapruder film.

The concepts developed in Fiester's book are established by specialists in ballistics and forensic analysis. You, instead, offer a paper by Ken Rahn whose field of speciality is atmospheric physics and oceanography, and who admits to undertaking his analysis with a conclusion already established in his mind.

This is of a pattern: previously you rejected the opinions of a senior ballistics expert in favor of concepts developed by J Edgar Hoover's urologist.