operational amplifier - What does this op-amp circuit do? (part of an ECG)

I would like create an ECG circuit based on this schematic (from the AD620AN datasheet):

I don't know this part of the circuit and how it works. I know this is called a right leg driven circuit which is reducing the effect of the noise. But I don't know exactly how negative feedback works in this case. Can someone help me?

Answer

The right leg driver tries to drive the average voltage of the body to cancel out noise. The right leg is chosen because it is far from the heart, so any signal injected on there will be common mode to two electrodes near the heart.

The right leg drive is much more tightly coupled to the body than ambient noise it picks up from capacitive coupling to things like the AC power in the room.

The network in the feedback path of the right leg driver opamp provides some low pass filtering of the signal.