In the paper ["Normal Subgroups in the Cremona Group"][1], it is stated that the induced isometry $f_ast$ of $f\in J_d$, where $J_d$ denote the set of Jonquiéres transformation of degree $d$, satisfies the equation $$f_\ast [H]=d[H] - (d-1)[E_{p_0}] -\sum_{i=1}^{2d-2}[E_{p_i}]$$ I understand the part where they say that $f$ has a base point $p_0$ of multiplicity $d-1$ and $2d-2$ distinct points of multiplicity 1 each. However I do not understand how they derived the above action of $f_\ast$ on the line $[H]$. Would be grateful if anyone point me in the correct direction! Thank you. 


  [1]: https://link.springer.com/content/pdf/10.1007/s11511-013-0090-1.pdf