The withdrawal or "pull out" method is a form of (not so great) birth control where the  man withdraws his penis from the vagina just before he ejaculates in the hope that entry of the ejaculate into the vagina and cervix, and therefore eventual fertilization, will be prevented.

Does it really protect against pregnancy?
The effectiveness of the withdrawal method depends on the male's ability to withdraw before ejaculation. Unfortunately, and because this method requires great self-control and experience, this is not a very effective or recommended birth control method for several reasons: The pre-ejaculate (pre cum) often contains a lot of sperm to cause pregnancy. And because the pre-ejaculate can enter the vagina and cervix before his orgasm and actual ejaculation, enough sperm will enter the vagina or spill on the vulva, so pregnancy is possible.

Disadvantages of Withdrawal
There is a high risk of pregnancy and STI exposure. Withdrawal does not provide protection from STIs, including HIV, and infectious diseases can be transmitted by direct contact with surface lesions and by pre-ejaculatory fluid, especially herpes or HIV. Withdrawal also interrupts the act of sex and there is no guarantee that a man can "pull out" in time and pre-ejaculate can contain enough sperm to cause pregnancy.

Pregnancy failure rates under typical use are 25 percent.

Advantages of the Withdrawal Method
It is always available. So in case a couple did not use adequate protection against pregnancy, using the withdrawal method probably decreases the risk of pregnancy.