A TEST OF YOUR TIMING
There's no science involved in the withdrawal method, probably because it pre-dates science as we know it. It simply involves the man removing his penis from the vagina before climax. It requires a lot of self-control and trust, and therefore it can be a pretty unreliable method of contraception. Not all sperm are released when a man ejaculates, so even if he pulls out before orgasm, you could still get pregnant.
The withdrawal method involves a lot of control on the part of the man, who must be aware of when he is reaching climax so he pulls out when ejaculation can no longer be postponed. Pre-ejaculate, or 'pre-cum', contains some sperm, and as such your doctor or nurse will likely not recommend it as a method of contraception.