  We report the enhancement of individual harmonics generated at a relativistic
ultra-steep plasma vacuum interface. Simulations show the harmonic emission to
be due to the coupled action of two high velocity oscillations -- at the
fundamental $\omega_L$ and at the plasma frequency $\omega_P$ of the bulk
plasma. The synthesis of the enhanced harmonics can be described by the
reflection of the incident laser pulse at a relativistic mirror oscillating at
$\omega_L$ and $\omega_P$.
