It essentially prevents pregnancy by preventing implantation during a specific time frame, as I understand the science.
This is not science but quackery.
Pregnancy begins at fertilization, not implantation. Most top selling medical dictinaries define it as either beginning at fertilization or just after (which is a few days prior to implantation).
Preventing implantation is an abortion, and you don't prevent or reduce abortion with earlier abortion.
The links below deals with this issue in depth, and examines how the abortion rights movement has tried to redefine pregnancy and conception in order to be able to say preventing implantation is not an abortion:
No Room for Contraception's Pregnancy Page -- clearing up the confusion