So I just made Heston's mushroom stock, which is delicious!
He cooks the mushrooms in butter until the water from the mushrooms evaporates.
Deglazes with wine. Adds aromatics.
Adds water.
Pressure cooks for 30 minutes.
After the pressure cooking step, he strains.
The he adds the liquid back to the pressure cooker, and reduces it until half in volume.
I don't understand the last part, why reduce? Why not just add less water in the pressure cooking step?