  We consider the dynamics of an XY spin chain subjected to an external
transverse field which is periodically quenched between two values. By deriving
an exact expression of the Floquet Hamiltonian for this out-of-equilibrium
protocol with arbitrary driving frequencies, we show how, after an unfolding of
the Floquet spectrum, the parameter space of the system is characterized by
alternations between local and non-local regions, corresponding respectively to
the absence and presence of Floquet resonances. The boundary lines between
regions are obtained analytically from avoided crossings in the Floquet
quasi-energies and are observable as phase transitions in the synchronized
state. The transient behaviour of dynamical averages of local observables
similarly undergoes a transition, showing either a rapid convergence towards
the synchronized state in the local regime, or a rather slow one exhibiting
persistent oscillations in the non-local regime, where explicit decay
coefficients are presented.
