"Diode circuits are used to shift the DC level of a wave form. If a signal has passed through a capacitor, the DC component is blocked. A clamping circuit can restore the DC level. For this reason these for units are sometimes called DC restorers." cited from Pearson Custom Electronics Lab manual pg.11.
To see what happens close the top left switch and you shall notice that the green sine wave jumps slightly above phase of the blue sine wave. Next you should switch the pole on the dpst switch below the left diode so that the power supply will be creating a lift for the sine wave to move further away from the blue sine wave. Play with the voltage and watch it rise and fall. Then switch that diode to open and close the reversed biased diode circuit on and do the same. This sine wave should switch to the negative portion of the blue sine wave. Hope you learned something new like I did in this experiment. : )
