Lines 7-12 Summary

Get out the microscope, because we’re going through this poem line-by-line.

their hips and their bellies off balance
to turn them. Kicking and rolling

Apparently, the speaker wants to make sure that we have a clear picture of what these dancers' bellies look like, because he (and we just assume it's a he, since we have no other indications) tells us about them again.
Here, he also mentions their hips and talks about how they're "off balance" as they turn.
All this body imagery paints a clear picture in our minds of free-wheelin' nature of this dance.
Notice too how the syntax is freewheeling here, with "to turn them" coming awkwardly at the end of the actual sentence (though at the start of line 8). "Them" actually refers to the dancers (as in, "to turn themselves"), but the reference is all the way back in line 2.
The twisting of the language really nicely fits with all the goofy twisting and shouting going on at the dance. These guys are far from trained dancers, but they're having a blast. When the speaker describes the dancers as "Kicking and rolling," we get an even more visceral image of just how wild and carefree this dance is.

about the Fair Grounds, swinging their butts, those
shanks must be sound to bear up under such

We get a nod to the setting when the speaker tells us that we're at "the Fair Grounds." (More on the setting can be found over at… "Setting.")
Then before you know it, we're getting more imagery of the dancers' bodies, complete with the image of swinging booties.
The speaker also notes that the dancer's shanks must be pretty darn sturdy ("sound") to deal with all this craziness.
The use of the word "shank" is kind of interesting. In case you don't know, it means the part of the leg between the knee and the foot. Though most humans, of course, have shanks, the word is more commonly used to describe cuts of meat, like lamb shanks for example.
So, the use of the word here might just be a sneaky way getting across the idea that these dancers' legs are thick and meaty. It might also bring to mind the wild, almost animalistic, atmosphere at the celebration.
Finally, we get more internal rhyme to weave the lines closer together: "butts" and "such."

rollicking measures, prance as they dance
in Brueghel's great picture, The Kermess.

The use of the word "measures" here reminds us the rhythmic nature of the poem and the dance it's describing.
However, the word "rollicking" reminds us of just how wild the whole thing is.
The speaker throws in more internal rhyme—"prance as they dance"—which us gives a sense of finality before we head into the last line.
Speaking of the last line: did you notice that it's the same as the first? What's up with that? Was WCW just being lazy?
Nah, we figure he's bringing us full circle. The rhythms and language of the poem has been so swirling and circular-feeling, it makes sense that he'd bring us back around to where we started. (Check out "Form and Meter" for the full breakdown.)
And with that final swirl... the dance has ended.