When a man is ready to ejaculate, his body undergoes a series of events.
He will witness an increase in the size of the head of the penis and it may also change its purplish color. His Cowper gland will secrete "pre-cum" fluid that will drain from his urethra. His testicles move towards his body and increase in size. In addition, he may experience redness, muscle tension, increased heart rate, and increased blood pressure.
Just before ejaculation, he will feel contractions in his vas deferens, seminal vesicles and prostate, causing seminal fluid (ejaculation). collect in a pool at the base of his penis. He will feel a "tickling" sensation. When he is ready to ejaculate, he will feel a "pulsation" around his urethra.
Ejaculate will leave his penis at roughly the same rate as an urban bus, about 28 km / hour. But can reach speeds of 43 miles per hour depending on how long since its last time. (Everyone is having fun until someone loses an eye !!)
Once he has ejaculated, his scrotum and testicles regain their normal size. He will feel a general feeling of relaxation and will go through a refractory period (he is physically unable to get another erection). This period can range from a few minutes to a lot longer depending on his age.
Sex tip: Unless it is about to return, you can prevent it from ejaculating by firmly squeezing (but very gently) the tip of the penis.