One of the challenges associated with making biopotential measurements is
rejecting the common-mode voltages to which the human body, electrodes, and
cables are exposed. That is the reason why having an instrumentation amplifier
with high CMRR is important.
Capacitive coupling of the 50/60Hz mains voltage is the main source of common
mode interference. You can picture this situation as a patient (so far
electrically floating) connected to a 220Vrms at 50/60Hz voltage source via a
low value capacitor.
This interference intensifies when the user/patient is close to electrical
equipment (a hospital bed with lots of bedside monitors, a surgery room…),
close to a motor (in a treadmill, for instance) or the system uses long cables
(susceptible to higher coupling).
One way of reducing this interference is by actively driving the body back with
a known voltage so that the body is not electrically floating any more. In its
basic implementation a dc voltage could be driven but the results are improved
if the body is driven back dynamically depending on the interfering common
voltage. This is done by constantly monitoring the changing common voltage the
two electrodes are riding on top of. This voltage is mirrored around the
reference voltage (usually half the supply) and driven back to the body.
In summary, as the body is capacitively pulled away from the reference voltage,
the RLD amplifier pulls it back in. It is traditionally named Right Leg Drive
because the driving is done in the part of the body which is furthest from the
heart (the right leg).