Video plethysmography (vPPG) is a noninvasive, remote diagnostics method that monitors cardiac activity by measuring subtle variations in the optical properties of skin driven by heart pulsations. However, the origin of the fluctuations in skin color at the heart rate responsible for the vPPG signal is not well understood. Using optical coherence tomography (OCT), we show evidence that the optical attenuation coefficient of the outermost layer of the epidermis is modulated at heart rate, being the modulation surrogate for the mechanical changes in the skin. We propose a hydraulic shock hypothesis to explain the phenomenon. The mechanical modulation of the non-vascularized epidermis would explain the modulation of optical properties of superficial skin, allowing for detection of pulsation even for non-penetrating radiation wavelengths such as blue light, as it is often observed.