  The local structure and electronic properties of Rb$_{1-x}$Fe$_{2-y}$Se$_2$
are investigated by means of site selective polarized x-ray absorption
spectroscopy at the iron and selenium K-edges as a function of pressure. A
combination of dispersive geometry and novel nanodiamond anvil pressure-cell
has permitted to reveal a step-like decrease in the Fe-Se bond distance at
$p\simeq11$ GPa. The position of the Fe K-edge pre-peak, which is directly
related to the position of the chemical potential, remains nearly constant
until $\sim6$ GPa, followed by an increase until $p\simeq 11$ GPa. Here, as in
the local structure, a step-like decrease of the chemical potential is seen.
Thus, the present results provide compelling evidence that the origin of the
reemerging superconductivity in $A_{1-x}$Fe$_{2-y}$Se$_2$ in vicinity of a
quantum critical transition is caused mainly by the changes in the electronic
structure.
