I am inclined to agree with Bill that there could be a broken stud shank in there.
I have scanned the relevant pages in my workshop manual and will give word for word the description of how the thing works. It is too difficult to scan the description without dismantling the manual, and that is a major job in itself!.
http://i17.photobucket.com/albums/b74/Peterff/CCF23092013_0000.jpg
http://i17.photobucket.com/albums/b74/Peterff/CCF23092013_0001.jpg
The description is as follows-(my manual is in English)--Drawbar 10 serves to secure the tool in the spindle 1. The threaded end of the drawbar is engaged with nut 8. As the nut is turned, the drawbar is axially moved in the spindle clamping the tool shank 16 through the medium of balls 15. To transfer the rotary movement of the nut 8 into the translator motion of the drawbar 10, use is made of the key 17 mounted on the spindle 1.
The nut is turned by flywheel 3. As the flywheel revolves, two levers 30 arranged in its rim are caused by the centrifugal forces to turn on pivots 19 and engage the projection of bushing 7 keyed onto nut 8.
The flywheel 3 derives rotation from motor 4 through elastic coupling 5.
When the motor and, consequently, the nut 8 rotate in the opposite direction, the tool is unclamped.
This is the vertical spindle. The horizontal spindle closer works the same way, the separate electric motor drive the drawbar through a bevel gear.
There are drawings for the horizontal closer which I could scan if this would help but the description of the workings of it are virtually word for word with the vertical spindle.
You can see that when the drawbar is in the unclamped position the balls can move outward thus letting the pull stud past them. It is quite ingenious albeit complicated. As yet I have had no problems with it.