The answer would be: preventing the movement of sperm into the uterus
Diaphragm is a birth control device that physically protects the uterus so the sperm couldn't enter it. Since sperm cannot enter the uterus, there will be no fertilization.
The diaphragm is shaped like a cup and needs to be put on uterus before the intercourse. A spermicide cream could also be added to kill the sperm that somehow passes through it.