How does the Bias Drive in TI ADS1299 help in noise reduction?
The body acts as an antenna for 50Hz or 60Hz radiation.  This can mess up low level signals and make it difficult to read out the ECG information.  A bias drive (also called Driven Right Leg) takes a copy of the 50Hz or 60Hz radiation and drives the leg with a scaled, inverted version, effectively canceling it out.  So, with a bias drive, the 50Hz or 60Hz interference appears both on signal and common, so you take the difference and it is canceled.