Pulling out really isn’t rocket science, the man withdraws from inside you before climax to minimize the risk of sperm entering the vagina and therefore reaching the egg. The key word is minimize, sperm still escape, and escaped sperm can still fertilize eggs. Men who use the pull-out method must be able to know when they are reaching the point in sexual excitement when ejaculation can no longer be stopped or postponed. If you cannot predict this moment accurately, withdrawal will not be as effective. Even if a man pulls out in time, pregnancy can still happen. Some experts believe that pre-ejaculate, or pre-cum, can pick up enough sperm left in the urethra from a previous ejaculation to cause pregnancy.
This is not a method that would be recommended by healthcare providers, or by the millions of people who tried it, put an early stop to the fun, and got unplanned pregnant anyway.