  We reveal a generic connection between the effect of time-reversals and
nonlinear wave dynamics in systems with parity-time (PT) symmetry, considering
a symmetric optical coupler with balanced gain and loss where these effects can
be readily observed experimentally. We show that for intensities below a
threshold level, the amplitudes oscillate between the waveguides, and the
effects of gain and loss are exactly compensated after each period due to
{periodic time-reversals}. For intensities above a threshold level,
nonlinearity suppresses periodic time-reversals leading to the symmetry
breaking and a sharp beam switching to the waveguide with gain. Another
nontrivial consequence of linear PT-symmetry is that the threshold intensity
remains the same when the input intensities at waveguides with loss and gain
are exchanged.
