Adiaphragm functions as a birth control device by preventing ovulation preventing the movement of sperm into the vagina preventing implantation of an embryo stimulating the production of sperm-killing antibodies preventing the movement of sperm into the uterus

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.

Allow large amino acids through