  We show that only those composite quantum systems possessing nonvanishing
quantum correlations have the property that any nontrivial local unitary
evolution changes their global state. We derive the exact relation between the
global state change induced by local unitary evolutions and the amount of
quantum correlations. We prove that the minimal change coincides with the
geometric measure of discord (defined via the Hilbert- Schmidt norm), thus
providing the latter with an operational interpretation in terms of the
capability of a local unitary dynamics to modify a global state. We establish
that two-qubit Werner states are maximally quantum correlated, and are thus the
ones that maximize this type of global quantum effect. Finally, we show that
similar results hold when replacing the Hilbert-Schmidt norm with the trace
norm.
