High Overlap Valve Timing & Reversion

[olddog]

I understand that at higher RPM the momentum of the exhaust gasses is used to start the air flow through the intake valve.

I also understand that at idle the intake manifold vacuum and perhaps a bit of exhaust pressure pulls/pushes exhaust gasses back into the intake, and we call it reversion flow. I had an intake gasket suck oil in, and there was an oil film all through the intake, out the throttle body, and onto the MAF sensor. So I know this is real, and have no dispute.

Top notch tuning experts say that high overlap valve timing also causes O2 sensors to get a false lean reading, at idle. They say that un-burned fuel/air mixture goes out the exhaust valve, and the oxygen is sensed by the O2 sensor, indicating a lean condition. However this is not a correct reading, because the fuel/air mixture was never burned. What is burning in the cylinder can actually be rich and still report a lean condition.

I'm a bit confused how exhaust gasses can be getting sucked back in through the exhaust valve at the same time some of the fresh air/fuel charge is exiting the exhaust valve. I suppose gasses can go one direction and then the other within the overlap period. Anyone ever hear of a theory on this?