There is some documentation on the various Nim backends here: http://nim-lang.org/docs/backends.html & http://nim-lang.org/docs/nimc.html#compiler-usage-generated-...

That second link describes the `nimcache` directory into which the generated C code is written. If you poke around in that directory (starting with the generated C version of your Nim code, which will have the same filename but a `.c` suffix instead of a `.nim` suffix) then you can see that the generated C code is really quite readable.