Ejaculation is a very complex process and nobody really knows exactly how it works.
https://en.wikipedia.org/wiki/Dapoxetine
Mechanism of actions[edit]
The mechanism through which dapoxetine affects premature ejaculation is still unclear. However, it is presumed that dapoxetine works by inhibiting serotonin transporter and subsequently increasing serotonin's action at pre and postsynaptic receptors[19] Human ejaculation is regulated by various areas in the central nervous system (CNS).[20] The ejaculatory pathway originates from spinal reflex at the thoracolumbar and lumbosacral level of spinal cord activated by stimuli from male genital. These signals are relayed to the brain stem, which then is influenced by a number of nuclei in the brain such as medial preoptic and paraventricular nulcei.[21] Clement's study performed on anaesthetized male rats showed that acute administration of dapoxetine inhibits ejaculatory expulsion reflex at supraspinal level by modulating activity of lateral paragigantocellular nucleus (LPGi) neurons. These effects cause an increase in pudendal motoneuron reflex discharge (PMRD) latency. However, it is unclear whether dapoxetine acts directly on LPGi or on the descending pathway in which LPGi located.[22]