This is "one of my favorites"! A very functional exercise that incorporates balance, flexibility, coordination, and strength. I especially like this for runners for the balance it requires on one foot while moving the other leg. Be sure to stand up tall at the top and contract the glute and hamstring of the back leg to bring the heel up as high as possible while maintaining good form. Good form includes shoulders back, chest out, low back and hips stable, as well as even hips throughout the exercise.
Have Fun!