Motion

You could argue that one of this poem's greatest strengths is the way it captures the motion of the dancers. No joke—when you're reading this poem it really makes you feel like you're watching these peasants really throw down. It might even make you feel like you're dancing with them. But how does the poet achieve this miraculous feat? Basically, he does it by having tons of fun putting just the right imagery in just the right places. Check it out...

• Lines 2-3: Williams kicks the poem off by telling us that "dancers go round, they go round/ and around," so already we've got a picture in our heads of these swirling dancers. This opening image works great to establish the whirling sense of motion that can be felt in both the rhythm and the rhyme of the entire poem. (Check out "Form and Meter" and "Sound Check" for more on that good stuff.)
• Lines 5-6: The poet pulls a nifty trick when he describes the dancers as "tipping their bellies (round as the thick-/ sided glasses whose wash they impound)." It seems like the whole point of these two lines is to compare the big, round bellies of the dancers to the big, round glasses they're drinking beer out of. Not only does he use a good old-fashioned simile (with the comparison using "as"), he also sneakily merges the two by saying that dancers are "tipping their bellies." You know, so it's like their tipping their big round stomachs up and down in the same way that they're tipping their glasses of beer into their mouths. Fun all around. 
• Lines 7-8: Williams hits us up with another great image when he describes "their hips and their bellies off balance/ to turn them." For one, it reminds us that the dancers are turning, bringing back the sense of circular motion from earlier lines. The idea of their lower bodies being a little out of sync as they turn also gives a clear idea of just wobbly this dance is. This line also has the wobbly syntax to match, ending with "them," which refers to the dancers mentioned 6 lines earlier. Hey, after all the beer these folks have tipped into their bellies, it's not much of a surprise that everything is a bit out of control. 
• Line 9: Just like the backup dancers of innumerable rap videos, the peasants in this painting aren't afraid of "swinging their butts." (Maybe they should call up Kanye.) Zeroing in on this particular part of the body doing its thing gives us a sense of just how into this dance these dancers are. Once the butt starts shaking, you know it's a party. 
• Line 11: When we hear the word "prance" it kind of makes us think of these Olympic dancing horses, but we're weird so whatever. When we're told that the dancers "prance as they dance" in this poem, it makes us think of how proud these dancers are. It's an interesting contrast to a lot of the other language in the poem, which gives a much more gritty sense of the dancers' movement. Here, we're reminded that these people are proud of their dancing skills. They may not be the richest people in the world, but their lives are still ones of dignity.