  Plateau inflation is an experimentally consistent framework in which the
scale of inflation can be kept relatively low. Close to the edge of the
plateau, scalar perturbations are subject to a strong tachyonic instability.
Tachyonic preheating is realized when, after inflation, the oscillating
inflaton repeatedly re-enters the plateau. We develop the analytic theory of
this process and expand the linear approach by including backreaction between
the coherent background and growing perturbations. For a family of plateau
models, the analytic predictions are confronted with numerical estimates. Our
analysis shows that the inflaton fragments in a fraction of an $e$-fold in all
examples supporting tachyonic preheating, generalizing the results of previous
similar studies. In these scenarios, the scalar-to-tensor ratio is tiny,
$r<10^{-7}$.
