Sound attenuation along a waveguide is a highly demanded research field, for applications ranging from heating and air-conditioning ventilation systems, to aircraft turbofan engines. Electroacoustic devices and digital control have provided the tools for crafting innovative liners where the boundary condition can be programmed. The most straightforward idea is to program classical local impedance operators. Nevertheless, it might be worthy to navigate off the beaten track, and try to target boundary operators which could never be physically produced by purely passive treatments. In this contribution, we focus the attention on a particular boundary law, called advective, as it possesses a convective character achieved thanks to the introduction of the first spatial derivative. After introducing such special boundary condition, we implement it on an electroacoustic liner and demonstrate its potentialities in enhancing the transmission loss in acoustic waveguides with flow. We present results for both plane and spinning modes attenuation. Numerical simulations and experimental implementations demonstrate the potentiality of this boundary law, opening the doors toward boundary operators involving spatial derivatives for wave propagation control.