trunk rotation slows down and arm accelerates. you don't want to spin through contact. many people now think that modern strokes are just relaxing the arm and then spin the body like crazy but this is not true. guys like fed have a great INDEPENDENT arm action. it's a multi stage rocket, first fires the rotation and then the arm as the rotation stops.
also get rid of the thought that the arm gets whipped through passively. the arm is accelerated by active contraction of the arm, shoulder and pec muscles. but this contraction must not be initiated too early or the kinetic chain is ruined. the muscles fire sequentially from the ground up. every ounce of muscle is used in a high level stroke but it has to be coordinated in the right sequence.