That will depend on your windbreaker. If your windbreaker was originally water resistant, than the Nikwax will help restore it, otherwise it does little.

I used on an old gortex jacket and it worked good for the season using both the Tech wash and Tx Direct. However it was different story when I used it on my pant. My 511 tactical had some Dupont coating that made it water resiliant, but it wored out after several washing. I tried the Nikwax Tech Wash, TX direct and the spray bottle letting it soak, and the result was ok ay best. The area where you get body to frantic rub will get soak eventually. Those area were places like the top of my thigh and knee. So I can imagine area like the shoulder or arms on a jacket will be where water will get pass the Nikwax.