"The major mechanism of action is that it prevents sperm from getting up into the uterus," Dr. Mary Jane Minkin, clinical professor of obstetrics, gynecology, and reproductive sciences at Yale School of Medicine tells BuzzFeed Life. It also thickens the cervical mucus, making it a very hostile environment for sperm. "It basically kills sperm on contact," says Minkin. It's also possible that hormonal IUDs might stop the release of an egg, but that's not the primary mechanism.
HOWEVER, if by some strange accident sperm does make it up there and somehow meets with an egg and fertilization occurs, it's possible that the presence of the IUD would prevent implantation. How? "We're not 100% sure. Many people think it's an inflammatory response," explains Minkin. Or it could be that the IUD thins the lining of the uterus, making it harder for an egg to attach. This doesn't mean that it kills the tissue — it just wouldn't implant, and you would have a normal period. That said, no small children would have been harmed in the making of that period. "What it would be is a collection of cells," says Minkin.
Worth noting: Lundberg and others also characterize the IUD as an abortifacient (meaning it can induce an abortion), which is based on the fact that it prevents implantation. In order for that to be true, one would have to define life at the exact moment an egg is fertilized. However, it's widely believed in the medical community that pregnancy doesn't occur until a fertilized egg has been implanted in the uterus.