The loop of the strncpy function is not understood

char *strncpy(char *dest, const char *source, size_t n) {
  char *start = dest;

  while (n && (*dest++ = *source++)) n--;
// I don't understand from the code below this
// I know I put'\0' at the end of dest.
// But I don't know how if (n) while (--n) works.
  if (n) while (--n) *dest++ = '\0';
  return start;
}

http://www.jbox.dk/sanos/source/lib/string.c.html#:38