Someone from Admob actually e-mailed me about these, he didn’t really explain what they did, this is an exact quote from the e-mail “ANO is a feature that allows AdMob to dynamically compete with your other ad networks listed in the mediation chain.”
I tried it on a few of my apps that have 100,000+ impressions a day and before I was getting about 97% fill rate, with ANO on, I get 100%. So must be good I guess. Using it on all my apps now and revenue has increased about 20% in the last 3 days. So give it a try and report back.