Ardha uttanasana (half stretch pose) seems to be a pose that I see done incorrectly so often. Because this pose is part of the Sun Salutation sequence, it is a common pose in many classes. It is not a difficult pose with immense sensation to discover. However, I feel as though this pose is not given the time and space for it do be taught or practiced correctly.
The point of the pose is to lengthen the hamstrings and to strengthening the back muscles. If this pose is done with a curvy spine the work is being put in straining the back, and that is the last thing that we want to do. Avoid locking in the knees. Engage your quads, lift the knee caps and find the tailbone and crown alignment.
I like having my feet hip distance apart with my knees slightly bent. Placing my hands on my shins, I activate my spine by engaging my extensor muscles. Ensuring that your neck is free, use your latissimus dorsi muscles to bring the shoulder blades away from the ears. A common error I see is long legs with a curvy spine with the hand on the shins. I like to focus on giving the cue, "Feel your tailbone go to the back of the room and the top of your head towards the front of the room. Make sure your back is flat enough to balance your brunch on there."
Below is a video of how I lengthen the hamstrings and strengthen the back in ardha uttanasana, as flow through the Sun Salutations. I invite you to press play and come play! Namaste.