Due to 19nor's being a Progestin they bind to the PgR (Progesterone Receptor), and nolva being similar to Extrogen binds to the Estrogen receptor and blocks it.

But..

By doing that it tricks the body into thinking there's both estrogen and progesterone present so the end result is higher prolactin. Which is a bad thing.

Using an AI is the preferable choice as it avoids that situation.

And caber or bromo to lower Prolactin.