Seated Calf Raise

Trains the Soleus.

How to do it

Sit in the machine and place the balls of your feet on the platform with your heels hanging off the edge.
Set the thigh pad snugly across your lower thighs, just above the knees.
Release the safety catch and let your heels drop below the platform until you feel a stretch through the lower calf.
Push up onto the balls of your feet as high as you can, keeping the knees bent throughout.
Pause at the top, then lower under control to a full stretch and pause briefly there too.

Bouncing through a short range. The calf's range is short to begin with, and the Achilles tendon's elasticity does much of the work if you let it bounce. A pause at the bottom removes that and makes the muscle start from a dead stop.
Setting the thigh pad too high, on the middle of the thigh, so the load is not transferred properly through the knee.
Skipping it because standing raises feel more productive. The bent knee is what makes this the soleus exercise, and the soleus is the larger of the two calf muscles — a calf routine without it has left the bigger one undertrained.
Rushing repetitions. The soleus is dominated by slow, fatigue-resistant fibres, so it suits higher repetitions and longer time under tension than most muscles.