Ejaculation is the process by which sperm from the testicles flows through the penis and is ejected out of the body. Here is an educational look at the penis ejaculating underwater, which gives a unique perspective of how the process occurs. First, the penis must be aroused by sliding the loose skin on the shaft back and forth. Watch the testicles tighten and rise at :32 seconds...at this point, ejaculation begins and couldn't be stopped no matter what the man does. Some pre-ejaculate begins to escape from the head and is immediately followed by an involuntary pumping action of sperm into the surrounding fluid. The white cloudy semen is ejected in a rhythmic series of blasts which cannot be stopped and will continue until the ejaculation is over. After all the sperm is expelled, the spent erection still "pumps" a couple times in an effort to empty the entire penis of all sperm. Hope you enjoy!