Often called standing splits, I usually refer to it as a one-legged forward bend (Eka Pada Uttanasana) as the focus should be not on lifting the leg as high as it will go and instead should be on maintaining alignment of the pelvis and extending through the legs. This is a great pose to challenge your balance, as well as stretch and tone the legs.
Begin in Warrior III with the right leg lifted. Keeping the hips parallel with the floor, lean forward to bring the hands to the floor. The tendency here is for the right hip to hike open and the thigh to rotate externally. Additionally, the left thigh and knee tend to rotate inwardly, which can put the knee at risk. It doesn’t matter how high you get the foot. See if you can keep the hips level so that the right toes point straight down toward the floor and keep the left knee pointing in the same direction as the toes. Iyengar says to keep the toes of the right foot pointed, though I usually flex the foot and reach strongly back through the heel. This helps to keep awareness on keeping the leg extended and the toes pointing down.
Keep both legs straight and hug the left femur with the muscles of the thigh (hamstrings and quads), gently lifting the kneecaps. For those that are very flexible in the hamstrings, you can take the ankle of the standing leg in either one or both hands (both = more of a balance challenge) and gently pull the forehead to the shin. Ground through the standing foot as you try and distribute your weight equally between all corners of the foot.
Stay in the pose for as long as feels comfortable (at least 20-30 seconds) and then repeat for the same amount of time on the second side.
If you have any questions, please leave a comment below. I’d love to hear from you or see you at one of my classes!