Butt wink you mean, when squatting? I'll make a vid. on friday to check but as far as I know I dont have much of a problem there.
I need more flexibility for sure when it comes to DLs and squats but I dont think thats the cause of this hunch(lower)back.
Buy some fat grips if you want bigger forearms, swear mine have gotten a tiny bit bigger after a few session. Did 5x10 with 60kg on deadlifts using them today. Was absolutely brutal. It's weird the weight starts moving really slow as your forearms get sore, being less then 30% of my max 60kg should fly up, but it started moving really slowly off the floor.