The "Reversing Beam" magic could nullify the effects of other magic, which was in itself an interesting reversal:
Reality being influenced by magic was the established, already-occurred fact. To reverse a spell's effects, one had to use magic to fabricate a "reality without magical interference" to cancel it.
In truth, a "reality without magical interference" had never existed in the first place.
Rorschach was in the practice room, attempting to apply the concept of "Order" to the extraction of Transmuting Dust.
Rorschach, of course, also wanted to reproduce the unbelievable magic that "could nullify a spell's effects." However, it was clear that the author, a practitioner of Chaos Evil, had not left behind any details on how to cast it. Apparently, the annotator—presumably a Great Mage himself—had also failed to reproduce it and had left a discouraging note.
