The dolly zoom, also referred to as the Vertigo effect or a Zolly shot, is a technique wherein the camera is dollied either forward or backward while the zoom on the lens is pulled in the opposite direction. When timed correctly, the effect of this technique is one in which the characters in the frame remain the same size while the foreground and background become compressed or de-compressed, depending on which direction the camera is traveling. It's a technique that has been part of the cinematic language for almost 60 years, and as such, it has evolved over time. In order to explain how it really works, a popular YouTube personality Michael Stevens of VSauce posted this interesting 12-minute video yesterday explaining the optical science behind the "Moon Terminator Illusion". Enjoy!