The term Coitus Interruptus literally means “interrupted sexual intercourse” and, in the classical sense, refers to the withdrawal or pulling out of the penis from the woman’s vagina during sexual intercourse, just before ejaculation of semen occurs. The man’s semen is subsequently directed to somewhere else in order to avoid the woman becoming pregnant.
Coitus Interruptus is perhaps one of the oldest methods of birth control. It’s actually not a very secure way of preventing pregnancy, although fairly successful if applied consistently, and failure rates may vary between 5% and 30% yearly, depending on a range of factors.
Mind that technically spoken Coitus Interruptus is not a method of Semen Retention. By contrast, semen is not retained at all, but deliberately ejaculated someplace else.
Nevertheless, withdrawal of the penis or pulling it out can be used as a form of Ejaculation Control or Semen Retention. It’s then typically called the Stop-Start Technique.
That is, by timely stopping thrusting movements and pulling the penis out when one feels the urge to ejaculate, sexual excitement can be reduced and ejaculation can be (or may be) delayed or avoided. After the urge has passed, one can start thrusting again until the next Stop-moment.