This exercise is one of the best ways to dynamically stretch the hamstrings. Keeping your toe dorsi-flexed and your leg straight, simply raise and lower your leg. Your opposite leg remains bent, as this takes some stress off of your lower back.
I like to increase my ROM during the set. The first couple of reps are done with a comfortable ROM and is increased as the reps are done. I like to perform a minimum of 20 reps per leg for at least 2 sets.