What I think it means is use 1 qt water per lb grain to achieve a step temperature of 131F
Then add 0.5 qt water per lb grain to achieve a step temperature of 152F. (The 0.5 plus the original 1.0 = 1.5 qt/lb thickness)
Then do a decoction (No extra water) to raise the temperature to 168F for a mash out.
I would be inclined to skip the 131F rest completely, and go straight to the 152F rest with a mash thickness of 1.25 qt/lb, and then add 6 qt boiling water to reach the 168F. (This assumes a 10 lb grain bill).