  A lattice mismatch between Van der Waals layers produces a moir\'e pattern
and a subsequent electron band reconstruction. When the bilayer is charged, the
sliding motion of one layer with respect to the other produces electric
pumping. Here I discuss the reciprocal process: that a voltage bias produces a
layer-shear mechanical force. The effect is deduced from the lowest-order
correction to the mechanical action by the coupling with electrons in an
external field. In twisted bilayer graphene the new mechanical force is shown
to be perpendicular to the applied field (due to C$_2$ symmetries exchanging
the layers) and proportional to the charge density measured from neutrality.
This is strictly true when the chemical potential is within a gap opened by the
moir\'e potential due to a topological quantization, and approximately true
when the chemical potential crosses the flat bands in a model including the
self-consistent Hartree interaction. In a mechanical device the effect should
be manifested as an apparent enhancement of the friction between layers when
the system is charged. Depinning fields for the sliding motion of the layers
are estimated in the order of $\mathcal{E}_c\sim$ kV/cm around the magic angle.
