Flow exiting a convergent-divergent nozzle operated at severely overexpanded conditions, such that a shock wave is formed inside the nozzle, exhibits a strong instability that causes mixing enhancement in the flow itself and can destabilize an adjacent flow. The latter property enables mixing enhancement of an arbitrary jet via axial injection of a secondary gas flow. A systematic investigation of this phenomenon reveals that the instability is associated with shock-induced nozzle flow separation. In this paper we review key results that demonstrate the potential of this mixing technique and provide insight into the relevant physical processes. In addition, we establish criteria for the occurrence and location of the separation shock, and construct correlations between mixing enhancement and shock strength.