When an intense electromagnetic wave is incident obliquely on a sharply bounded overdense plasma, strong energy absorption can be accounted for by the electrons that are dragged into the vacuum and sent back into the plasma with velocities v\ensuremath{\simeq}${v}_{\mathrm{osc}}$. This mechanism is more efficient than usual resonant absorption for ${v}_{\mathrm{osc}/\mathrm{\ensuremath{\omega}}g\mathrm{L}}$, with L being the density gradient length. In the very high-intensity ${\mathrm{CO}}_{2}$-laser--target interaction, this mechanism may account for most of the energy absorption.