[en] The relation between the macroscopic input power required at ASDEX Upgrade to access the H-mode $P_{\mathrm{t}\mathrm{h}\mathrm{r}}$ and the microscopic $\mathbf{E}\times <!--\; ??\; -->\mathbf{B}$ shear has been investigated via fast charge-exchange recombination spectroscopy (CXRS) measurements at various toroidal magnetic fields, different electron densities, and in both hydrogen and deuterium plasmas. For the H-mode onset, a threshold in the $v_{\mathrm{E}\times <!--\; ??\; -->\mathrm{B}}$ minimum, an approximation of the $\mathbf{E}\times <!--\; ??\; -->\mathbf{B}$ shear, has been found. This identifies $v_{\mathrm{E}\times <!--\; ??\; -->\mathrm{B}}$ and not E _r as the important player for the L-H transition. A database of measurements including CXRS, Doppler reflectometry measurements and comparison to neoclassical approximations shows a threshold $v_{\mathrm{E}\times <!--\; ??\; -->\mathrm{B}}$ of (6.7 ± 1.0) km/s ranging over a factor of three in $P_{\mathrm{t}\mathrm{h}\mathrm{r}}$. Using these findings, a simple derivation of the $P_{\mathrm{t}\mathrm{h}\mathrm{r}}$ scaling is proposed giving a physics interpretation of the B _t, density and surface dependence of $P_{\mathrm{t}\mathrm{h}\mathrm{r}}$. (paper)