Contract your hamstrings to lift the weight up to your bum, try to keep your hips down as you lift.
Slowly lower the weight under strict control. This is where the real benefits of this exercise are.
TIP: Many people feel their calves cramp up on this exercise. That’s because they put their calves into a stretch position (imagine pulling your toes up towards your knees), which fatigues the muscle and burns like f**k. If that’s happening to you, try moving the machine pad up a notch so you can perform the exercise without putting your calves on stretch.