The Soleus as a Knee Extensor, ACL tear prevention

I recently purchased a spider squat bar. You see, ever since I found out these things existed, I've wanted one. Well, I finally bought one, and it is now hanging proudly in my power rack in my bedroom-gym.

Only the coolest bedroom in the world...

Spider squatting is quite different from your typical squat (<--- best squat article ever, by the way), as the bar always seems to want to pitch you forward, making this extra awesome if you want to train core stability.

Anyway, I started practicing doing some reps past parallel with this 80-pound beast, and I noticed something that I had forgotten about when reversing direction coming out of the hole... the fact that the soleus muscle (the deeper of the two calf muscles) can be used in close chain activities as a knee extensor.

How is this possible?