I love heights under certain conditions. As long as I am wearing my climbing harness that is tied into one end of the rope and the other end of the rope is secure in a bomber anchor, I feel good. If I am not tied in or if I have any doubts about my anchor, my stomach turns, I get tense, and cranky. When rappelling over the side of a cliff, the scariest moment is when there is still slack in the section of rope between me and the anchor, I have to step backward and lean over the edge of the cliff to put tension on the rappelling rope.
The tension in that rope is critical in allowing me to precisely control the rate at which I descend to the ground. Once the rope is loaded by my mass leaning over the edge, the only way to relieve the tension in the rope generated from my mass, is to disconnect the load from either side or to cut the rope. Either scenario would cause me to fall. So, with me hanging on one end and the other fixed to an anchor, there is life and limb-saving tension on the rope. Once I reached the bottom, I can untie myself from the rope. With only the mass of the rope pulling down, the rope is slack.
What does this have to do with tight hamstrings? Consider how the hamstrings lengthen as one moves into a forward bend from a standing position, i.e., eccentric hip flexion. In this scenario, the role of the hamstrings is analogous to the role of the rope in rappelling. Here's the set up: 1) The distal ends of the hamstrings are anchored to the proximal tibia and fibula (the anchor) while the other ends attach to the ischial tuberosity (my harness). 2) The load on the hamstrings as one performs a forward bend is the mass of everything above your ischial tuberosities (e.g., your core, upper body, and most of the mass of your pelvis). The load in the rappelling scenario is about the same. The hamstrings are like the rappelling rope in that they require tension to control the rate of lowering the load. Granted, the intracellular mechanism that allows the hamstrings to change length while maintaining tension is certainly different then the mechanical characteristic of a rope, but in these two scenarios, they are doing the same mechanical job: providing the tension needed to control the rate of descending mass.
So, appropriate tension in the hamstrings is a necessary feature of proper function. How much tension should there be in the hamstrings? Is there a difference in the amount of trainable motion from person to person? Why do hamstring stretches feel so different from person to person? What would prevent a person from being able to touch their toes or move into Uttanasana? The complexity of the answers to these questions may surprise you. It all has to do with the specific issues in the tissues. Stay tuned...