It simply changes its y-coordinate based on the function. It is continually updating the graph. So when you move the sliders, the graph updates instantly. The program multiplies the cubic term by .001, the quadratic term by .01 and the linear term by .1 so that more of the graph is likely to fall within the bounds of the screen. You can also graph functions using the patches.
X become green.
If so, I should turn green. Similarly, to decrement the slider, click to the left of the handle. If the new value is outside the boundary of the screen, the program hides the turtle.