In normal operation of a MOSFET, current flows through a thin channel between the drain and the source. Voltage applied to the gate turns this current on and off.
When the very high voltage of an electrostatic discharge (ESD) pulse arrives at the drain, the current leaving the drain becomes extremely large (1) because of avalanche multiplication. This current flows down through the substrate to the back contact (2), which changes the voltage in the substrate until the transistor "snaps back" and the current again flows to the source (3). Generally the maximum heating occurs at the junction of the drain and the channel.