This is a subject I have been very confused with lately about thrust coming from downwash on a propeller and helicopter, but to understand how the downwash increases thrust is to know how it forms and works on a regular aircraft wing. A picture of downwash on a wing would be helpful. And also how does downwash make thrust on a propeller and helicopter wing?
Having in mind a typical flow around an airfoil, the flow over the wing flows much faster than the one under the wing. This faster particles arrive first to the trailing edge and are "projected" against the rest of the flow, creating that additional thrust. Moreover, the difference between the free stream and the flow after the trailing edge is the downwash angle. I hope it helped.