Han gör inte
I Förräderi av Isengard diskuterar Christopher Tolkien kortfattat denna passage; Han medger att han har svårt att förena de motstridiga berättelserna, men föreslår möjligheten att de tre vandrare såg en vision av Saruman, projicerad från Gandalfs sinne:
Against Gandalf's words my father wrote in the margin: Vision of Gandalf's thought. There is clearly an important clue here to the curious ambiguity surrounding the apparition of the night before, if one knew how to interpret it; but these words are not perfectly clear. They obviously represent a new thought: arising perhaps from Gandalf's suggestion that if it was not Saruman himself that they saw it was a 'vision' or 'wraith' that he had made, the apparition is now to emanate from Gandalf himself. But of whom was it a vision? Was it an embodied 'emanation' of Gandalf, proceeding from Gandalf himself, that they saw? 'I look into his unhappy mind and I see his doubt and fear', Gandalf has said; it seems more likely perhaps that through his deep concentration on Saruman he had 'projected' an image of Saruman which the three companions could momentarily see. I have found no other evidence to cast light on this most curious element in the tale
The History of Middle-earth VII The Treason of Isengard Chapter XXIV: "The White Rider"
Vad än det är som de såg, vare sig de härrör från Saruman eller Gandalf, är Tolkiens samförstånd att det inte var fysiskt, och det var inte nödvändigt för Saruman att ha fysiskt reste till skogen.