Smith Machine Calf Raise

Trains the Gastrocnemius.

How to do it

Set a block or a step under the bar of a Smith machine.
Set the bar height so you can get under it with the balls of your feet on the block and your heels hanging off.
Rest the bar across your upper back as for a squat and unrack it.
Stand tall with your knees straight, and keep them straight for the whole set.
Let your heels drop below the block until you feel a stretch through the calves.
Push up onto your toes as high as you can, pause at the top, then lower under control to a full stretch.

Bending the knees, which shifts the work to the soleus and makes it a different exercise.
Not using a block. Without something to drop the heels below, half the range disappears and the stretch — the useful part — never happens.
Bouncing out of the bottom, letting the tendon do the work instead of the muscle.
Loading it heavily and shortening the range. The bar's fixed path makes heavy loading easy; the range is what makes the set worth anything.