Per the instructions, and personal experience, instead of lowering it straight down, kind of push it toward the back of the machine so it catches the notch. In this position the foot is a slight distance from the feed dogs and has put some tension on the threads so that the hook can grab the loop at the back of the needle without pulling bunches of thread out. The distance that the presser foot is from the feed dogs keeps the fabric from pulling up with the needle. I've heard many times of people not putting the presser foot down at all when FMQ and wound up with globs of thread all over. Pfaff made these machines so that there is room to move the fabric and still be able to make an actual stitch.
Janey - Neat people never make the exciting discoveries I do.