You made me doubt what I was doing ;-)

The best way to activate your soleus involves plantar flexion or pointing your toes downward, while your knees are in a bent (preferably at or around 90 degrees) position. Bent knees during heel raises target the soleus. If you do not bend your knees, the larger more powerful gastrocnemius muscle will be activated.

So it's definitely as described. Knees bent, feet flat, raise the heel, toes pointed down, shorten the soleus muscle, heels back down. Repeat. Repeat. Repeat.