# Pearlmutter's method for Hessian multiplication

I am trying to understand the abstract below from Pearlmutter's paper.

Can someone clarify to me why $R_{\bf{v}}\{\bf{w}\}=\bf{v}$? Thanks a lot!

$$\mathcal{R}_\mathrm{\textbf{v}}(\mathrm{\textbf{w}})\equiv (\partial/\partial r) (\mathrm{\textbf{w}}+r\mathrm{\textbf{v}})=\mathrm{\textbf{v}}$$