Several questions recently posed by Inelastic Electron Scanning Tunneling Spectroscopy (STM-IETS) experiments are clarified. First, the method of calculating inelastic currents as a function of the tip's position and the bias is shown, as well as how to relate the currents to experimental results. We apply the method to calculate the rotation rate profile of the acetylene molecule on Cu(100) as a function of the tip's displacement. The fraction of electrons inelastically scattered by the different vibrational modes of the molecule is calculated also as a function of the bias, in a range of several volts. The results of the calculation explain why the C-H bending modes were not detected in STM-IETS experiments.