  We study an unusual but robust phenomenon that appears in an example system
of four coupled phase oscillators. We show that the system can have a robust
attractor that responds to a specific detuning between certain pairs of the
oscillators by a breaking of phase locking for arbitrary positive detunings but
not for negative detunings. As the dynamical mechanism behind this is a
particular type of heteroclinic network, we call this a 'heteroclinic ratchet'
because of its dynamical resemblance to a mechanical ratchet.
