How does Progestin Only emergency contraception work?

The EC doesn’t protect you if you have unprotected intercourse after taking EC.⁵¹

If a woman uses Progestin-only Emergency Contraception within the first 24 hours of unprotected sex, her risk of pregnancy is reduced by 95%.  Overall, Progestin-only Emergency Contraception can reduce the risk of pregnancy by 88%.   The longer a woman waits to use Emergency Contraception, the less effective it is. For each woman, it depends on where she is in her cycle when she has unprotected sex. For example, if a woman is about to ovulate, she may risk pregnancy if she waits 5 days to take EC. If a woman is already ovulating (the egg is in the fallopian tupes) than the Emergency Contraception may not work even when taken right away.⁵²

Emergency Contraception doesn’t prevent pregnancy from intercourse that occurs after its use.