Can someone explain this rear brake?

The frame with the wheel isn’t supposed to bolt onto the top of the arm. It’s supposed to feed the cable thru the silver nub that’s sitting in the end of the link on the left and then the cable gets clamped to the top of the arm on the right.

Edit: googled some images of “v brake wheel” and I may be wrong. What I’ve always seen is stuff like the “travel agent” which runs like I described but I did find another pic of a part like you’re showing being run like you show but on a canti brake. So, I donno, you have an odd one there.

Yes it changes the ratio – by virtue of the off center bolt hole on the single pulley, creating a cam.

The travels agents you often see referenced do the same thing but with two separate concentric pulleys with the cable jumping from one to the other.

This appears to be setup with a competitor of the “Problem Solvers Travel Agent” pull ratio adapter. It doesn’t look like the Problem Solvers part, but the design appears to do the same thing.

Since this bike has a cantilever brake levers that are integrated with the shifters, this was the only way to make the v-brake work without buying new brake AND shift levers (shifters are expensive, the adapter is cheaper).

… I am confused why they left the cantilever brake up front, though. Odd. I would’ve expected matching v-brakes front and rear with those same adapters on both. But, I’m OCD and like my brakes to work the same front and rear. 🤣