  A conventional superconductor sandwiched between two ferromagnets can
maintain coherent equilibrium spin current. This spin supercurrent results from
the rotation of odd-frequency spin correlations induced in the superconductor
by the magnetic proximity effect. In the absence of intrinsic magnetization,
the superconductor cannot maintain multiple rotations of the triplet component
but instead provides a Josephson type weak link for the spin supercurrent. We
determine the analogue of the current-phase relation in various circumstances
and show how it can be accessed in experiments on dynamic magnetization. In
particular, concentrating on the magnetic hysteresis and the ferromagnetic
resonance response, we show how the spin supercurrent affects the
nonequilibrium dynamics of magnetization which depends on a competition between
spin supercurrent mediated static exchange contribution and a dynamic spin
pumping contribution. Depending on the outcome of this competition, a mode
crossing in the system can either be an avoided crossing or mode locking.
