Weighted False Grip Pull-Up

Set your grip with your wrists rolled over the top of the bar rather than hanging straight down from it, so the bar sits low in your palm near the base of your fingers instead of across them. It's an uncomfortable grip at first -- most of the strain shows up in your wrists and forearms before your back even gets involved. Pull up while keeping that wrist position through the whole rep; letting your grip roll back to normal partway up defeats the purpose. Lower with control rather than letting the bar pull your wrists back straight. Expect your grip to fatigue well before your back does.