The text mimics the sensations/descriptions/emotions (etc.) of the passage. Furthermore, it's supposed to challenge the reader and recreate the elements discussed in the text (i.e. swirling textual passages represent vertigo, narrowing passages represent claustrophobia, etc.). It's all very carefully and meticulously planned by Danielewski. And if you can get by the pretentiousness and frustration, it all comes together wonderfully. I was seriously in a state of mental exhaustion for about a week after reading it.