Not clear to me fully how, but they claim to:

CFD technology from ANSYS was used to predict fluid flows around the
body of an elite swimmer in the outstretched glide position (assumed
immediately after the initial dive and following each lap’s turn off
the pool wall) to identify areas where drag, and its slowing effect, is
likely to occur. The images shows flow pathlines colored by local flow
velocity. Image courtesy of ANSYS, Inc.

CFD Review | Speedo Improves High Performance Swimsuit with ANSYS Software