The way I was always taught was that when you're doing them right, the bar should be damn near tearing the skin off your shins. You want to keep your core engaged until it passes your knees and then fire out with your hips which will straighten out your back. It's really your lower body that should be driving the lift. Your back shouldn't even be stressed until the final like fifth of the pull. The bar should just be going straight up and down. Never had any injuries, so I put a lot of stock in it.