The GTO causes a reaction called the inverse myotatic reflex- skeletal muscle contraction causes the antagonist(opposite) muscle to simultaneously lengthen and relax. So by contracting the hamstrings, we are using the nervous system to help us relax the quads. It's not a perfect system, but you get the idea. By combining a nervous system reflex, the stimulus from rolling, and the mechanical friction on the muscle we can get a better response to your foam rolling.
The video example is of foam rolling the quads, our knee extensors. We start by putting the muscle in a shortened position, in this case by extending the knee. Then as we apply pressure just above the kneecap we slowly flex our knee as we roll up the thigh.
**As always, remember to roll toward your heart as you roll out your arms and legs. Visit the post Foam Rolling Techniques from a Massage Therapist and refer to Rule #2.
Archer, P. (2007). Therapeutic massage in athletics. Philadelphia: Lippincott Williams & Wilkins.