
I believe this back squat pic is taken from the same workout.
Notice how Platz is leaning forward some. By leaning, in addition to the knee joint, the hips are also a lever and a fulcrum, hence the term "compound movement."
In front squats, the upright position, that Ritch mentioned, takes the hips out of the lever equation, and puts the main burden in the knee joint. You can get away with this for awhile, but eventually, you'll most likely have knee issues.
Of course, if you lean too far forward doing back squats, you'll have the same issue only in the lumbar area.