C 9x9 sudoku solver

Hey /prog/
could you guys explain how do the "safe" and "solve" functions work in this code? http://pastebin.com/eYQYdUB7
or just add comments next to each line? Anything is fine, really.

why is there a return x; after every if?
if the first if in the function safe is true then it returns TRUE and doesn't do the following if's and for's, right?