The manual is here.
http://www.manualowl.com/m/Pfaff/76/Manual/333723
Darning starts on page 18.
I had to look it up. The Pfaffs of that age don't seem to have that second presser foot position.
All it says is "Lower the presser bar lifter"
In light of that, I would compensate with less pressure on the presser foot if that's an option. I didn't see it in my brief look through the manual either.