1. Start on your back with your arms at your sides
2. Lift your hips up
3. Alternate back and forth bringing your heels towards your glutes
4. Keep your heels firmly pressed into the ground

You can go pretty quick here as long as you can maintain your form. You can use your arms to aid in stability but make sure your hips stay off the ground. This exercise fires the hamstrings eccentrically and it really fires the hamstrings so be careful how much volume you do with it.