HOW: Get set-up standing with a box or object in front of you that you can put one foot up on. The side that is on the box will be the side targeted. While maintaining body position, lift the foot off the box as high as you can driving your knee up towards the ceiling, slowly lower and repeat.
FEEL: You will feel the hip flexor muscles working on the side being worked as well as your hip muscles on the other side working to maintain single limb stance stability.
COMPENSATION: Do not lean side to side, do not round or arch your back, try to isolate hip motion.