  Recently, there has been some vigorous interest in the out-of-equilibrium
quasistationary states (QSSs), with lifetimes diverging with the number N of
degrees of freedom, emerging from numerical simulations of the ferromagnetic XY
Hamiltonian Mean Field (HMF) starting from some special initial conditions.
Phase transitions have been reported between low-energy magnetized QSSs and
large-energy unexpected, antiferromagnetic-like, QSSs with low magnetization.
This issue is addressed here in the Vlasov N \rightarrow \infty limit. It is
argued that the time-asymptotic states emerging in the Vlasov limit can be
related to simple generic time-asymptotic forms for the force field. The
proposed picture unveils the nature of the out-of-equilibrium phase transitions
reported for the ferromagnetic HMF: this is a bifurcation point connecting an
effective integrable Vlasov one-particle time-asymptotic dynamics to a partly
ergodic one which means a brutal open-up of the Vlasov one-particle phase
space. Illustration is given by investigating the time-asymptotic value of the
magnetization at the phase transition, under the assumption of a sufficiently
rapid time-asymptotic decay of the transient force field.
