How to release tension from extension spring that won't unstretch

Hi,

I'm learning how to replace one side of the extension spring/cable/pulley system for a basic Clopay manual garage door. (Not a torsion system.)  I encountered a peculiar issue. When opening the door, I found that the door became very heavy somewhere between my belt and my chest height. When inside, I saw that the cable that runs up vertically from the bottom of the garage door toward the pulley that rests on the metal framework near the ceiling (forgive the lay person terms) was stuck and not rolling over the pulley.  In fact, I think it fell off the pulley and was on the metal framework. Over time, it had even cut a groove into the metal!  (See photo.)  

The upshot of this is that the extension spring didn't slacken or unwind as the door opened, like it should.  I researched how to replace the cable, and which type to buy.  And I had C clamps and everything.  My hope was I could take the spring off its J hook at the very top of the ceiling/framework,, and thereby reduce all tension. But the spring is still too stretched because of the cable issue described above. I need the Hulk to help me pull the spring off the hook. 

The only other solution would be to cut the cable somewhere.  But of course I'm EXTREMELY AWARE of how much tension and force is hiding in that spring, and not only would the spring slam "shut" at great speed, but various bits of slack from the cut cable could fly in any which direction like a live wire. I wouldn't feel safe doing this unless it was with 15 foot cable cutters lol.

Any advice on how to kill the tension and remove all these parts, so that I can move forward and replace the pulley and cable like I planned?

Thanks!!

Thanks! I found a video showing this exact procedure, then learned to my dismay that my arrangement (installed by previous owners) doesn't include this winding cone. The spring is just attached on the back end to a J hook, which is very secured to the metal framing.

Anyway, what I ended up doing was grab my huge leather wood stove gloves, put them on, stood outside my garage, and reached in with long handle wire cutters, and I cut the cable that way. No one was harmed! And I was able to then install the new cable at the right length and tension and back on the pulleys.

Thanks!