When I first started training with Phil, I looked at his workout... "Hmmmm?" I thought. Then I tried it... Ah! I had to brush off some dust off of the Kinesiology portion of my brain... motor unit recruitment.
Simplified version
1) Say you have 100 muscle fibers.
2) 10 motor units (basically nerves) attach to those 100 fibers. 1 motor unit equals 10 muscle fibers that it activates (flexes, contracts)
Here's the thing... you never really activate ALL motor units, meaning, as much as you are contracting/lifting, you are not activating all your muscle fibers.
The goal- activate as many motor units as possible, to activate as many muscle fibers as possible.
As I think through this method, it makes sense to me now why the weights might be easier on the concentric and why it causes growth...
1) By pushing down on the eccentric you are activating more motor units.
2) Then, on the concentric, you now have more muscle fibers "activated" or "recruited" to contract during the movement.