There are two versions for the M104.
One is a large wrench which holds the pulley. The other is a bent rod that you have to insert through the back of the pulley where there are designated holes. There is a notched area on the hub mount that the rod slips through in order to keep it from turning. That's a horrible description, but you know what works great on this site? The search function. TONS of write ups on this procedure.