From AMS Glossary
The archetype of mechanical mixing is the instability of a vertically sheared flow. Consider a flow with an upper and a lower layer flowing in opposite directions. If the flow is unstable, perturbations grow by transferring momentum (through Reynolds stresses) between the layers and reducing the velocity difference between them. The kinetic energy of the shear flow (proportional to the mean-square velocity) is therefore reduced, and the energy has gone into the perturbations. These perturbations grow to the point where they become turbulent and cause vertical mixing.