For forward bends like this, we can categorize them as restorative in part. Of course, they are about the hamstrings lengthening functionally, but they are also restorative in the sense that the head goes below the heart. At least half of our body is upside down and therefore changes the effort required to generate blood flow, at least to half the body […] Now, you’ve flipped it upside down and gone restorative which is associated with the parasympathetic nervous system.
Once you have either of these two positions, don’t initiate all of the work from the arms. Rather, use the arms to enhance the length in the spine. What I often encourage students to do first is to soften their abdomen. Yes, you read that correctly, relax the abdomen. Why? By relaxing the abdomen (which can be more difficult the tighter your hamstrings are) you will be forced to use deeper muscles. Which muscles? Well, the psoas comes to mind as the strongest of all of the hip flexors. Many students are so well trained to use their abdominal muscles that the body almost defaults to this muscle.
When the abdomen contracts, it’s going to try and flex the spine. Of course, some flexion is okay, but if we’re also trying to lengthen the spine, the tension in the abdominal muscles will actually work against this since it’s trying to bring the ribs toward the pubic bone. With the abdomen more relaxed and the psoas engaged, the hip flexion becomes the primary action happening.