A few things to remember as you do this one...
First: if your forehead and knee are not touching, your posture has not begun yet. Touching forehead to knee is essential to achieve the proper compression on the internal organs and on the thyroid gland. The good news is, you can bend your knee as much as you need to in order to get the forehead on the knee!
Often as you bend forward toward your knee, the hips can twist out of line, so make sure that you apply weight to the front leg to get your both hips level.
Benefits of this posture include slimming of the abdomen, waistline, hips buttocks and upper thighs. Also by compressing pituitary gland, thyroid and para-thyroid glands this posture helps to balance your endocrine and immune systems.
One of the best ways to learn more about the postures is to read Bikram's "Beginning Yoga Class" book. Attend class 3 to 5 times weekly to put your learning into action and make your body smile!