Keep your legs straight without locking your knees.
While inhaling, lower the weight of the hack machine.
Continue going down until your calves reaches an angle lesser that 90 degrees.
Maintain your knees inline with your toes. Hold for a count of one.
Now while exhaling, push your legs up and return to the starting position and stay there till a count of one.
Repeat the above steps for reaching the desired number of reps.