The calf muscle, also know as the triceps surae, translates in Latin to the "three headed muscle of the calf." It is composed of the 2-headed gastrocnemius muscle and the soleus muscle. Both muscles are some of the strongest in the human body, attach to the Achilles' tendon, and are the primary ankle plantarflexors. However, there is one extremely important distinction between the two that is often overlooked when stretching the calves, and that is that the two muscles originate on opposite sides of the knee joint and thus must be stretched differently.
The gastroc attaches above the knee and is a knee flexor, so when your knee is extended and you stretch the calf with ankle dorsiflexion, you are primarily stretching the gastrocnemius.
The deeper soleus muscle attaches below the knee and thus is NOT a knee flexor. Therefore, to stretch the soleus, YOU MUST SLACKEN THE GASTROC FIRST by flexing the knee. Only once the gastroc is on slack, can you effectively stretch the soleus muscle with ankle dorsiflexion. A tight soleus muscle is implicated in many foot/ankle orthopedic conditions and stretching of this muscle is many times overlooked.