Do not lock your knees out at the top of the reps.
Lower the weight slowly.
Make sure your knees do not track out over your toes as this will cause undue stress to the knees.
Keep your head up throughout the exercise - dont look down.
Instructions
Set up the hack machine by loading the weight you want to use.
Position yourself with your back flat on the pad and shoulders up against the shoulder pads.
Position your feet in a narrow stance.
Push up to take the weight off the stack, place your arms on the side handles of the machine and disengage the safety bars. This is the starting position for the movement.
Slowly lower the weight down until your thighs are at approximately right angles with your calves.
Pause, and then push the weight back up to the starting position without locking your knees at the top of the movement.