The simple answer is that orgasms occur in the brain, ejaculation in the prostate and urethra, and they are two different things that generally but not always occur together. Actually, it’s not that simple because orgasms also involve the SNP and involuntary contractions of the anal sphincters and other muscles, in the groin and elsewhere, but the simple version captures the main idea.
In brain scans, an orgasm looks remarkably like an epileptic attack, a thunderstorm that spreads over the brain. Actually, it begins at “the point of no return,” followed by one to three seconds later by the onset of ejaculation.
The key point is that you can have one without the other. Men who have experienced anorgasmic ejaculation (without sexual arousal or orgasm) say it is not entirely pleasant. Men who experience orgasm without ejaculation are divided. Some (mostly young people) say that a non-ejaculatory orgasm is somewhat less pleasant or intense. Others (mostly older men) say they are equally nice. Even so, it is quite clear that most of the pleasure comes from orgasm itself.
The other important point is that there is no loss of erection if you have an orgasm but do not ejaculate. It is ejaculation, not orgasm, that spills hormones into the bloodstream that causes detumescence. Therefore, being able to reach an orgasm without ejaculation increases the resistance of man and allows him to have many more orgasms.
When Hartman and Fithian conducted their monumental study of human sexuality (more than 10,000 hours of recordings of people who masturbated and had sex while connected in the laboratory!), They found that approximately 12% of their male subjects could have Orgasms without ejaculating and without losing your erections. These 33 men averaged 4 orgasms per session (usually half an hour or so) and one had 16 orgasms in less than an hour.
Some links that may be useful:
Myths about male orgasm
Types of male orgasms
Men, orgasms and ejaculation.
Multiple orgasms for men
In some interviews with experienced Tantric men, about 40% of men may choose to have an orgasm without ejaculating, probably because men who are motivated enough to learn Tantra are more likely to be motivated enough to learn this trick as well.