What it is
Hexagrams have six lines, but Activity and Passivity each have one more text. In the I Ching a moving yang line is called a "nine" and a moving yin line a "six," after their line values. Hence the names: 用九, "using the nine," and 用六, "using the six."
What they say
- In 1: "Dragons are seen in a throng, none at the head. Good fortune." All that strength in motion, with no one forcing their way to the front.
- In 2: "Lasting constancy is favorable." All that receptiveness in motion, held steady over time.
When to read it
When all six lines of one of these two hexagrams are moving. All of 1 changes into 2, and all of 2 changes into 1: each becomes its opposite. That's exactly when the extra text makes sense: strength that knows how to yield, receptiveness that knows how to last.
Other hexagrams with six moving lines have no extra text; the most common rule, from Zhu Xi, is to read the judgment of the relating hexagram.
How rare it is
With the three-coin method, getting 1 with all six lines moving happens once in 262,144 casts. If it comes up, it's worth lingering over. More on the rules in Moving Lines.