I don't have the double release cable with me now, so I will write this from memory. My head is getting worse and worse, so take it with a bit of care
The end that threads into the body has a kind of adjustment: you could loose a second threaded part and tighten it again leaving a longer or shorter tip. Effectively, what you are doing is increasing or decreasing the time difference between the moment the body mirror is "released" and the lens shutter is released. The longer the body tip is, the bigger time separation.
This explanation has nothing to be with your problem, but just in case