By "structure," Dair means the different letter forms of
different kinds of typefaces.
Which means you choose your emphasis word, then put that word
in a completely different typeface than the rest of your text.
The use of structure involves contrasting the thicks and thins of different typefaces to achieve distinct visual differences between the thicks and thins.
The quotation demonstrates how emphasis can be controlled. The word "architectural" contrasts with "sculptural" and by doing this, the structure change is not only like increasing or decreasing the volume, but changes the very quality and tone of the voice to suit the expression.