In the article Mori Dream Spaces and Blow-Ups by Ana-Maria Castravet, I found the following claim:
A birational map $\varphi:X -\to Y$ between normal projective varieties is called contracting if the inverse map $\varphi^{-1}$ does not contract any divisors. If $E_{1},\ldots E_{k}$ are the prime divisors contracted by $\varphi$, then $E_{1},\ldots ,E_{k}$ are linearly independent in $N^{1}(X)$ and each $E_{i}$ spans an extremal ray of $\textit{Eff}(X)$.
She cites the article Mori dream spaces and GIT by Yi Hu and Seán Keel. In this article, they give some comments with respect to this claim but there is no proof. I would like to know if it is true and if there exists a reference where I can read the proof or if you can tell me an argument.
Thank you.