ECG Shield electronic circuit explanation
The  first op amp sets up a reference at Vcc/2 for single supply operation.  The resistors on the input limit current to the body and set up a body reference at or near the common mode voltage on the electrodes (using big noisy resistors).  The instrumentation amp takes the differential signal, provides a gain of 5, and spits it out as a single-ended output,  where the next op amp low-pass filters it, and then sends it to an inverting amp with a big gain (1M/3.3k) that also removes most of the DC offset prior to amplification.