I thought EC just prevented implantation - but on a government website, I found this:
"Emergency contraceptive drugs appear to work primarily by preventing or delaying egg release (ovulation) from the ovaries. They may also slow egg or sperm transport in the fallopian tubes, and they may make the uterine lining less hospitable for implantation of a pregnancy."
http://www.nlm.nih.gov/medlineplus/ency/article/007014.htmAlso, they pointed out something which I had forgotten - that an IUD put in within 5 days after unprotected sex also works as EC.