As well as accurately predicting rates, the instanton approach provides an intuitive mechanistic interpretation. The table-of-contents graphic depicts the instanton pathway at 300 K with the paths traveled on the reactant and product surface colored in blue and red, separated by the "hopping point" shown in purple. We find that the carbon atom contributes about 75% of the squared mass-weighted tunneling path length, sulfur about 10% whereas the remainder is from the two chlorine atoms. The reaction rate corresponding to this instanton is more than 1000 times faster than the classical rate, which is a dramatic speed up considering that this arises from tunneling of heavy atoms at room temperature.