  We numerically study the spin flip in the Frisch$\unicode{x2013}$Segr\`e
experiment, the first multi-stage Stern$\unicode{x2013}$Gerlach experiment,
within the context of the novel co-quantum dynamics theory. We model the middle
stage responsible for spin rotation by sampling the atoms with the Monte Carlo
method and solving the dynamics of the electron and nuclear magnetic moments
numerically according to the Bloch equation. Our results show that, without
using any fitting parameters, the co-quantum dynamics closely reproduces the
experimental observation reported by Frisch and Segr\`e in 1933, which has so
far lacked theoretical predictions.
