The hip flexors are used when kicking in all strokes and are a common area of tightness in swimmers and multi-sport athletes. Upright Dragon is a tremendous pose to stretch out the hips flexors and increase hip mobility.
By extending the arm on the same side as the back leg you target the largest hip flexor muscle, the psoas, with greater intensity.
Is a 500-hour trained yoga teacher and therapist. He began teaching yoga after a 20-year swim coaching career.