This paper discusses the dependence of the efficiency of the Doherty power amplifier, DHT PA, on its load-pull ratio, i.e., the ratio between the small-signal and the full-power load resistances presented to the intrinsic transistor of the carrier (or main) DHT PA. The study starts by explaining which are the physical mechanisms responsible for the observed drain efficiency dependence on drain resistance, ç(Rd), to then derive a physically consistent model to predict this ç (Rd) profile. It is theoretically demonstrated that the observed ç (Rd) maximum is due to the carrier PA transistor's knee voltage and soft turn-on that are responsible for the positive and negative ç (Rd) profile slopes, respectively. Finally, it is also shown how this model can be used to estimate the optimum load-pull path that should be provided by the DHT dynamic load modulation.