"Cold setting" (AKA bending) the frame. This is where I bend the frame so that it can fit a rear hub as big as the one I've bought (which is 132mm long between the locknuts).
Have I done this before? Of course not. An astute reader may wonder how I know what the hell I'm doing? Ahhh, I owe it all to Sheldon Brown, a man I've never met. He tells the world how to do things like "cold setting." And tons of other things. If it weren't for his website, I never would have started such a project. Certainly not with confidence.
So what does "cold setting" entail? You stick a piece of wood in there and press down gentle but firmly on the seatpost tube. Then the left (upper, in the picture) dropout gets pushed out (up, in the picture) away from the other dropout.
This spreads the dropouts (where the rear wheel goes). Initially, the dropouts were a (old) standard 128mm.
128 mm spread on the dropouts (the dropout you can't see is below).
I had to spread them to 132mm. That's how big the Shimano Nexus hub is. 2mm more on each side. The dropouts can easily be pulled apart that distance, but you have to give them a little force to permanently spread them. That way it's not a pain everytime you take the wheel on and off (like when you get flat).
After. Now we're at 133mm! Perfect.
The string is a way to make sure that the dropouts are evenly spaced from the center. After a few adjustments, they were.