Driven by cold atom experiments and a desire to understand far-from-equilibrium quantum transport, we analytically study spin-helix dynamics in one-dimensional $XX$ models. We use the Jordan-Wigner transformation to map the spin chains to the non-interacting Fermi gas with a simple equation of motion. However, the resulting dynamics are non-trivial, as the spin-helix initial conditions correspond to a highly non-equilibrium distribution of fermions. We find the timescale separation between the in-plane spin dynamics and the out-of-plane spin dynamics. We can gain insights from analyzing the case of uniform spin chains and semi-classical models. One of our key findings is that the spin correlation function decays as t^{-1/2}$ over time, in contrast to the experimentally observed exponential decay.
