Originally Posted by SafeAirOne
I don't know if your Pfaff is like my Adler, but mine has a feature where if the hook mechanism gets bound up or jammed for whatever reason (usually operator error in my case), a clutch mechanism will allow the hook assembly to break free of the drive.
The hook assembly will still spin, but it won't be in the correct phase with the needle till you lock it back in to its detent by holding the hook mechanism from rotating with one hand while you turn the handwheel with the other hand (don't do it using the pedal if you like your left hand).
EDIT: Just had a look at the Service Manual (not Instruction Manual) in the link Dan posted for the Pfaff 1245. It looks like the 1245 has a safety clutch similar to the one I described above. It's addressed in section 1.04.12 of the SM, including how to reset it. The reset method I described for my Adler may not work for this Pfaff.
Most industrial machines have a safety clutch. The method for re-setting the clutch varies from machine to machine. Mine requires I hold down a button on the bed of the machine while turning the hand wheel backward. Normally, when the clutch is disengaged, nothing moves. I have never jammed my machine up bad enough with just thread to require re-setting the safety clutch. I had a bent needle that stopped the machine in its tracks that required re-setting the clutch.