Hasselblad extension tube 56E (40656) is attached to the camera body in exactly the same way as a lens. The camera has to be rewound (drive shaft in proper position) :

The drive shaft in the extension tube should be cocked in the same way.

The tube has to be attached first to the camera body and finally the lens to the tube. Removal is made first with the lens, then the tube. If not done this way the shaft will uncock.

E in 56E means data transmission through data bus for 200 camera series. The tube gets contacts on both ends to transmit data from lens to camera body. So you keep full compatibility with FE lens.

Extension Tubes Diagram

Remarks :

Be carefull with adding and removing tubes. respect the order, or you risk to uncock the tube shaft and more seriously to jam the camera body.

To rewind the tube's shaft, just use a small coin and turn it clockwise back in place.

To un-jam the camera body is fairly more complicated. Go here to have explanations on how to do it.