Discharge utilizes the internal circuitry of the charger (resistors) to discharge the battery. You are typically pretty power limited in this case. Regenerative discharge allows you to basically convert the discharge power from a battery into charging power to another battery in a dual port charger or back through the power input to a supply battery. This method typically allows for much higher discharge rates versus using the internal circuitry. On a dual port charger, regenerative discharging can also allow for the use of a large resistor in place of a second battery to absorb the power of the battery being discharged.