Semileptonic $\tau$ decays: powerful probes of non-standard charged current weak interactions

When looking for heavy ($\mathcal{O}$(few TeV)) New Physics, the most efficient way to benefit from both high and low-energy measurements simultaneously is the use of the Standard Model Effective Field Theory (SMEFT). In this talk I highlight the importance of semileptonic $\tau$ decays in complementing, in this respect, the traditional low-energy precision observables and high-energy measurements. This is yet another reason for considering hadronic tau decays as golden channels at Belle-II beyond the unquestionable interest of the CP violation anomaly in $\tau \to K_S\pi {\nu }_{\tau }$ decays, that I also discuss within the effective theory. A couple of new results for ${\tau }^{-}\to K^{-}{\nu }_{\tau }$ decays are also included.