If fertilization does occur, Plan B may prevent a fertilized egg from attaching to the womb (implantation).

That's outdated information from 2009. It doesn't prevent, or help prevent, implantation.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5102184/

The drug was originally thought to work by preventing fertilization. Recent research has cast doubt on this. Our review of the research suggests that it could act in a pre-fertilization capacity, and we estimate that it could prevent ovulation in only 15 percent or less of cases. The drug has no ability to alter sperm function and limited ability to suppress ovulation. Further, data suggest that when administered pre-ovulation, it may have a post-fertilization MOA.