The causal law therefore is not so accommodating as to let itself be used like a hired cab, which we dismiss when we have reached our destination; rather does it resemble the broom brought to life by the apprentice-wizard in Goethe's poem, which, when once set in motion, does not leave off running and fetching water until the old master-wizard himself stops it, which he alone has the power to do. These gentlemen, however, have no master-wizards among them.
The particular context, which is a rather clumsy and unimpressive attack on cosmological arguments, is of less concern than the basic point of the claim, which is exactly right: principles do not let themselves be used as hired cabs, and you cannot simply ride them to whatever destination you please. This is one reason why in philosophy it is very misguided to focus only on a narrow set of problems: philosophy has more to do with principles than other fields, and once you commit to a principle, you are committed to it, period. A principle that seems fine when applied to this sort of problem may well put things massively out of joint elsehwere; a refutation that seems devastating if we consider this case only might well commit us logically to claims that have thoroughly absurd implications in yet another case.
But Schopenhauer would have made a better analogy if he said that the causal law is like the spell used by the apprentice, rather than like the broom. The broom could, in fact, be stopped; but the spell could only be nullified. And so it is with principles: Und non komm, du alter Besen, they say, and the brooms keep marching; and only when one finds the refuting word are things again as they were.