A great way to release tension on the hip flexors while engaging the abdominal wall is to work on the reformer performing leg circles. I usually start leg circles on a green spring. ( medium tension) and let my legs loose up while circling around while placing my hands on the hip flexors.
As I press gently down my thumbs onto the hips I create more ‘room’ between the head of the femur bone and the hip sockets. The stirring motion of the circles facilitates this task.
Then I am ready to add up an extra spring on the reformer and with my hips warmed up and loose I can work on strength while focusing on my hamstrings, inner thighs and lower abdominal. I like to think that legs start form the torso so I can concentrate on this feeling of control and energy flow while performing leg circles.