In ping pong operation you have two identical amplifiers each one mounted in a configuration allowing auto-zero of its input offset. Then, circuit operates cyclicly by zeroing one amplifiers during one clock perio, while the other is connected to the main loop, then, during the next clock period, the zeroed amplifier is connected to the main loop, while the other amplifier is zeroed.
As you swith from one amplifier to the other, while the second amplifier is being zeored, this is called ping-pong.