The curvilinear path in elite swimming is not an illusion caused by body rotation or elbow flexion. It’s undeniably present in the world’s fastest swimmers.
Here’s just one example of the incontestable proof shown in over 300 photographs in Swim Speed Strokes: Elizabeth Beisel swims butterfly in this photo sequence below. There’s no body rotation in butterfly swimming, yet we can clearly see her arms taking a curved path, not a straight path, as she presses back on the water.
Like all elite swimmers, Beisel’s pull is highly propulsive partly because she navigates to planes of ‘still’ or ‘non-moving’ water rather than pushing back on the same column of moving water. No train tracks here!