  The $\sigma$-machine was recently introduced by Cerbai, Claesson and Ferrari
as a tool to gain a better insight on the problem of sorting permutations with
two stacks in series. It consists of two consecutive stacks, which are
restricted in the sense that their content must at all times avoid a certain
pattern: a given $\sigma$, in the first stack, and $21$, in the second. Here we
prove that in most cases sortable permutations avoid a bivincular pattern
$\xi$. We provide a geometric decomposition of $\xi$-avoiding permutations and
use it to count them directly. Then we characterize the permutations with the
property that the output of the $\sigma$-avoiding stack does not contain
$\sigma$, which we call effective. For $\sigma=123$, we obtain an alternative
method to enumerate sortable permutations. Finally, we classify
$\sigma$-machines and determine the most challenging to be studied.
