My P/P remarks are directed to colleague TurbineD - but here's the elucidation: P/P is simply the total pressure ratio, in this case the tailwind upstream pressure divided by downstream pressure. In the tailwind case the number is very low, close to unity, and when applied to the rear end of the LPT won't do much.
But applied to the much larger fan blades, a substantial amount of torque can be had even at very low P/P.
Compris?
And deploying the reverser places the blocker doors in the way of this P/P, closing the valve, spoiling the driving force of the fan rotor.