  Using the recently introduced ACV reduced-action approach to transplanckian
scattering of light particles, we show that the $S$-matrix in the region of
classical gravitational collapse is related to a tunneling amplitude in an
effective field space. We understand in this way the role of both real and
complex field solutions, the choice of the physical ones, the absorption of the
elastic channel associated to inelastic multigraviton production and the
occurrence of extra absorption below the critical impact parameter. We are also
able to compute a class of quantum corrections to the original semiclassical
$S$-matrix that we argue to be qualitatively sensible and which, generally
speaking, tend to smooth out the semiclassical results.
