This exercise develops the soleus muscles. Unlike the standing variation, it doesn't bear excessive load on the lower back.
Sit down on the machine.
Place your lower thighs under the lever pad and your feet on the block lowering your heels down.
As you exhale, contract your feet and rise up as if your were standing on your toes.
Pause for 2-3 seconds at the top.
As you inhale, return to the starting position trying to stretch your calves to the maximum.
Do not jerk the weight up. Do the exercise slowly.
Do not grab the handles, otherwise you can unconsciously help yourself to lift the weight.
Lower your heels as low as possible to achieve the best range of movement.