This position is not a yoga pose per se, but rather has been borrowed from the strength and stretching-centered doctrine of pilates. However, pilates and yoga intersect in some major ways, and this position is certainly an important crossover.
The upward hamstring position concentrates on the stretching and strengthening of hamstrings. The flexibility of hamstrings is a very integral part of the practice of yoga, and improvement in yoga really depends on our hamstrings. A huge variety of yoga poses hinge on our ability to condition the strength of our hamstrings and also their flexibility. As such, mastering this position over time will lend to improvement in the practice of yoga. Begin lying on the ground with your back on the mat. Your legs should be stretched forward, your toes pointed upward, and your arms stretched above your head on the ground.
Take a deep breath and slowly raise your right leg off of the ground, bringing it as close to perpendicular to your upper body as possible. At some point, you will not be able to lift it upward any longer. At this point, you will reach both of your hands upward to grab hold of your foot. If you cannot do this because your hamstring is too tight, you can use a yoga strap (wrap the strap around the arch of your foot and grab hold of each of the strap ends in your hands). Lightly pull your foot downward with your hands so you can feel a tight stretch in your hamstring. Hold this position for about 30 seconds, and release back to the ground. Switch sides to stretch your left hamstring.
As time goes on, you will notice a huge improvement in your hamstring flexibility. You will be able to practice this position without the use of a strap, and you will notice your legs giving much less resistance. This is great, but always remember to challenge yourself.
Stretchy athletic pants are important when practicing yoga and pilates, as well as simple stretching positions like the one described above. Try these Puma athletic pants if you find yours to be too restrictive.