In EC, you can only measure potentials referring to the circuit's ground. Especially in complicated circuits, you might want to measure voltages across one single component. You could use the voltmeter component, but your results are as usefull as Kiani's comments.
So use this setup instead. Just hook up one of those things you never really knew what they did to your component, set it's gain to 1V/V and attach a ground to it's negative output. You can now scope the potential difference between the two inputs. In Germany we would say "Neiße Scheiße alter!" now.
The switch switches between improved and not improved measurement.
