We propose a method for inducing magnetization reversal using an AC spin current polarized perpendicular to the equilibrium magnetization of the free magnetic layer. We show that the critical AC spin current is significantly smaller than the corresponding DC one. The effect is understood as a consequence of the underdamped nature of the spin-torque oscillators. It allows to use the kinetic inertia to overcome the residual energy barrier, rather than suppressing the latter by a large spin current. The effect is similar to a swing which may be set into high amplitude motion by a weak near-resonant push. The optimal AC frequency is identified as the upper bifurcation frequency of the corresponding driven nonlinear oscillator. Together with fast switching times it makes the perpendicular AC method to be the most efficient way to realize spin-torque memory valve.