To remove/fit a lens to a Hasselblad, the body and lens must be cocked. In exceptional circumstances, you can get out of sync, meaning there is a problem, you can't cock the shutter, you can't fit a lens.

Solution is simple. There is a small inverse screw/slotted shaft within the body which needs rotating to re-cock the body/shutter.

This article goes into more detail of the problem including images.

This tool is handy to carry with you at all times, as the problem is most likely to occur when you want it least!

Note: The tool has two ends, the small end is used to wind the screw inside a body, and the large end is used to wind the screw inside the lens.