The following diagram shows that Rc5 may cause a considerable voltage drop.

To ensure that current does not flow through R1, an addition nail is used for the feedback of the guarding amplifier.

Since the current that flows in Rc1 and Rc6 is null, the guarding amplifier cancels the current that passes in R1.

The external guarding technique may be useful also in measurements with 2 wires, to put in guarding components with a very low resistive value.