Heat transport at the nanoscale is said to be affected by non-local effects that do not appear at the macroscale. A popular extension of the Fourier law that accounts for this effects is the Guyer-Krumhansl equation, \begin{equation} \tau\dot{\mathbf{q}}+\mathbf{q}=-k\nabla T+\ell^2(\nabla^2\mathbf{q}+2\nabla\nabla\cdot\mathbf{q}), \end{equation} where $\tau$ and $\ell$ are non-classical parameters which are called thermal relaxation time and phonon mean free path. The second derivatives of the heat flux are said to account for the non-local effects. However, I cannot find any explanation for why these effects are included by including these terms into the Fourier law.
Can anyone help me to understand the interprete these terms?