Actually, events beyond the plateau phase can be broken into many additional stages. My first sensations come from my testes, which draw up into my body. The ampulla filling with sperm accentuates sexual stimulation pleasure. Continued penile stimulation soon triggers a hardening of the seminal vesicles which discharge fluid into the ampulla then into the ejaculatory ducts. Sufficient pressure quickly builds in the ejaculatory ducts to trigger a hardening of the prostate, which discharges directly into the urethra.
Ejaculation is inevitable once the prostate hardens.
Finally, the valves at the ends of the ejaculatory ducts open simultaneously with the initiation of the ejaculation reflex. Note that there are valves at the ends of the ejaculatory ducts and that fluids pressure triggers the ejaculation reflex. I evidently orgasm and discharge sperm, ampullary gland, and seminal vesicle fluid into the ejaculatory ducts without triggering ejaculation or the opening those valves. I have trained myself to lock the ejaculatory duct valves closed during these emissions. This muscle relaxation locks the ejaculatory ducts valves closed, even as the seminal vesicles proceed to harden and discharge.