The goal of this stretch is to increase motion accross the front of your hip joint. This will in turn decrease pelvic forward tilt, increase freedom of movement in the knee joint and take pressure off of your hip.
Most people who are tight accross the front of the hip can not get into the position as shown in this illustration. Note how the spine is straight and not arched backwards (which would put unwanted pressure on your spine) and that the bent knee is straight down. The follwing two images show the CORRECT positioning for this stretch.
If you cannot grab your ankle and use the proper positioning, then rest your foot on a lower object, such as a chair (as in the pictures below). Once you get more motion, try a higher object.
