There is a tool made just for your problem. It's called a 'marlinspike' and is used by sailors to undo knots and bends when they've been drawn too tight.
You can find an example here.
As others have stated, the best bet is to attempt to add material into the knot/bend. This facilitates loosening the troubled area.
To explain further, from an end of the work, find the first crossing and, from the side opposite the end, insert the marlinspike (or corkscrew, screwdriver, what-have-you) between the end piece and the crossing (typically, under the crossing piece toward the end.)
This should allow you to make a little extra space in the first crossing's loop and then you can push some material from the end into the knot, thereby creating more slack in the knot / bend.
Continue to do this with all the crossings in the knot/bend until satisfied with the result.