A 33-year-old man presents with P. falciparum malaria (parasitaemia 2.5%) acquired in Tanzania. He is started on IV artesunate. Serial parasitaemia is monitored: 0 hours 2.5%, 12 hours 2.8%, 24 hours 1.2%, 48 hours 0.1%, 72 hours 0%. This pattern of initial rise then rapid clearance is consistent with which pharmacological property of artesunate?