The Rule
Give the reader some level of detail to visualize the scene you're painting.
Description
The passage used to describe a John Hughes-esque dance scene — "clothing, hairstyles and dance moves all indicate that this time period is the late 80s" — avoids any specific details. The phrase "a variety of '80s dance moves" isn't exactly expressed but came to be the shorthand for this trope later.
Counter-Example
"As Gregor Samsa awoke one morning from uneasy dreams he found himself transformed in his bed into a gigantic insect."
Counter-intuitively, when the very weirdness of the situation might incline the writer to lard up the sentence with explanatory clauses, the spareness works for Kafka, as the reader gets pulled in by the desire to learn more.
Commentary
M&C spend 30-45 seconds spontaneously describing a few fashion trends of the '80s to demonstrate how little effort it would've taken to flesh this phrase out a little.
As we'll see in a later rule, Mike & Conor will drag prose for being over-written and giving too much detail. Knowing how much or how little prose is necessary is one of the critical elements of being a writer, and it depends on knowing your readers. Cline clearly knows his readers.