  In this paper we consider the so-called procedure of {\it Continuous Steiner
Symmetrization}, introduced by Brock in \cite{bro95,bro00}. It transforms every
domain $\Omega\subset\subset\mathbb{R}^d$ into the ball keeping the volume
fixed and letting the first eigenvalue and the torsion respectively decrease
and increase. While this does not provide, in general, a $\gamma$-continuous
map $t\mapsto\Omega_t$, it can be slightly modified so to obtain the
$\gamma$-continuity for a $\gamma$-dense class of domains $\Omega$, namely, the
class of polyedral sets in $\mathbb{R}^d$. This allows to obtain a sharp
characterization of the Blaschke-Santal\'o diagram of torsion and eigenvalue.
