This function swaps the variables plotted along the horizontal and the vertical axes.
When you use Browse to draw 2 variables, you can't control which variable is plotted along the horizontal axis and which variable is plotted along the vertical axis. If you realize that you want the 2 variables to be plotted the other way around, use Switch XY. Sometimes, you may also want to switch the axis variables so that it'll be easier to compare a plot with other plots.
If there are zoom lenses aiming at the plot to which Switch XY is applied, all associated oculars will also have their variables swapped. Since an ocular is a viewport and can also be zoomed into, the above statement implies that the oculars of an ocular whose axis variables have just been swapped will also swap their axis variables.
You can also invoke Switch XY in an ocular, the axis variables in the plot at which the associated zoom lens is aiming will be swapped, too.