When the movement of x's in the "outer" circle begin to shift, it creates a sort of whirlpool effect. If you've made a whirlpool in the spa you might have noticed the center becomes a small, tight, whirling circle. In turn, the movement of water allows one to float on the outer layer of the water.
Just like the concept of floating around the edge of a whirlpool, the x's in the circle during a game of duck-duck-goose are doing the running while the people in the mush-pot are stuck sitting.
