The more you contract a muscle under resistance, the better the mind/muscle connection gets, the stronger the signals move along the pathways and the better the pumps will be when you train.
I've been training hamstrings for years without much success, then I realized that in comparison to a biceps contraction, my hamstrings contractions were weak - plain and simple I had been lifting too much weight to physically contract those muscles properly.
So I stripped it right back to basics, concentrated on time under tension with a lighter weight and focusing on strong peak contractions - and whammo - I now have growing hamstrings, and more importantly hamstrings that I am in control of.
