Pulling out on time usually refers to withdrawing the penis from the vagina before ejaculation (cumming). Pregnancy can occur at this time for two reasons. There are drops of pre-ejaculatory fluid that come out of the tip of the penis during excitement. These drops of "pre-cum" contain sperm and aren't usually felt when they come out. The second reason why this may cause pregnancy is because withdrawal requires excellent self-control. Often when a man is really excited, withdrawing before ejaculation may be next to impossible. Withdrawal is not an effective method of birth control. An alternate method is recommended.