The shaft is driven off the camshaft. Assuming the engine runs the camshaft is turning. The shaft is connected to the camshaft through an Oldham coupling of Perfo engines and a splined coupling on 11D engines.
I would imagine the change was made to avoid this happening – so is yours a Perfo engine?
This link goes to Jose Franssen’s page which shows how this goes together:
https://www.citroen-traction-avant.com/en/shop/article/S005/Cam_shaft
Either the coupling itself has broken or the dog has snapped off one of the shafts – hopefully not the camshaft.
Chris