reverser efflux does not enter the engine even with negative airspeed (i.e. a tailwind), meaning that the reversers can be used to back the aircraft using engine power alone.
Tailwind is your friend when backing using reversers. The idea is to keep the eflux from crawling far enough forward that it get's over-run by the inlet moving forward or caught in the suction of the inlet.
Lots of ways to manage this effect (forward speed vs eflux size, tailwind or aft speed to create your own tailwind)