[bobcowan]

It's not a theory, it's a fact. If the valve is open on the piston down stroke, it's going to creat negative pressure in the exhaust tract, and suck hot exhaust gasses back into the cylinder.

Well designed headers will create a negative pressure are at the collector, and that helps control reversion. IIRC, Doug Thorley used to build anti-reversion headers. GM get's around this by using a very wide LSA on a stock engine - around 115*.

The intake track gets contaminated on the upstroke, when the intake valve is still open, but the exhaust valve is fully closed.