Lie on back, bring feet close in to your body with knees bent, raise one leg in the air, and then bridge up with the other foot so that you are supported on just one foot and on your upper back. Head and neck should be loose, and shoulderblade area should do the majority of work in upper back. You can hold this position for a certain amount of time, or press up and down from the ground to complete a quantity of reps.
This is an amazing activation for hamstrings and for the lower third of your glutes.