When we twist the spine as in Ardha Matsyendrasana, (Half Lord of the Fishes Pose), we reach hard-to-get-to places in the body! The Posture was named after the famous Yogi Matsyendra, the co-founder and creator of Hatha Yoga from the 11th century CE!
The key to proper alignment for beginners in this pose is keeping the heel of your right foot (assuming the left leg is on the floor), right next to the left knee. Most beginners and some intermediate students have their right foot too far back so the toes of the right foot touch the knee. When you touch the left knee with the right heel it lets the body (especially the hips) open and allows you to grab the left knee easier. If restrictions in your hips prevent you from grabbing the knee, place a towel under your body and grab a hold of the towel, as close to the knee as possible. This is a more effective way than having your left arm dangling in the wind. Holding the towel creates leverage and will allow for a deeper release. If you are very inflexible and can’t sit with your left knee bent, simply straighten your left leg. Many professional athletes (especially football players) need this modification.