This is a natural method of birth control: withdrawing the penis from the vagina before ejaculation. It’s also called ‘coitus interruptus’ and ’being careful’.
Because the semen remains outside the vagina it decreases the risk of pregnancy. But it’s far from safe. If you use this method for a year you have 18-20% chance of becoming pregnant. Without using any method the chance would be 60-80%. So it helps, but you cannot rely on it: it’s far from safe.
As a girl you should never trust a guy who says he’ll be careful. Even if he withdraws in time you still run the risk of becoming pregnant.
The withdrawal method does not protect you against STD’s.

Why is this method unreliable?

Are we well protected if we use a condom and I withdraw before coming?