i think i figured it out.
progestins make cervix thick so sperm cant travel up. so i guess u take it before having sex.
antiprogestins terminate pregnancy, which means ur already pregnant and the blastocyst is sticking to the endometrium lining. Progesterone is needed for endometrium to be sticky. if u stop progesterone the blastocyst wont stick anyone and u add misoprostol (contract uterus) to eject the contents out