Complex sentence: If the machine does not halt, this algorithm will always reject: it must eventually terminate, since a computation path in which the machine continues writing digits forever is not fair, as the "go to the next stage" choice is never taken.
Split into simpler sentences: If the machine does not halt, this algorithm will always reject: the only way it could run forever is if it never leaves the number-writing stage. But such a computation path is not fair, since the "go to the next stage" choice is never taken.