I present detailed numerical experiments on a recently proposed S-wave generation method and show the excitation characteristics of the downgoing S-wave caused by a vertical impact acting close to a step-like topography. The geophysicists' group of Akita University recently studied and confirmed this phenomenon by in-situ measurements and named it the "pit effect." The numerical simulation is performed in 2-D space by the indirect boundary-element method using the Green's function for a homogeneous unbounded medium. The results show that the pit effect is a practical S-wave generator vertical seismic profiling (VSP) and refraction surveys. The generation of a vertical downgoing S-wave does not require any special machinery, so the pit effect can be used in many situations and especially in remote areas and difficult enviroments such as swamps and steep mountain ranges.