[Shane@DBPerformance]

Reversion is when the exhaust the an exhaust manifold or header starts going backwards/the wrong way. So people try to make the runners in the header bigger, which causes there to be a step up at the exhaust port in the head. The step is to try to slow down the exhaust gas from getting back into the combustion chamber. On a turbo car though, you have extreme amounts of pressure in the exhaust manifold under boost(usually 1 to 2 times the amount of boost/pressure in the intake manifold), so a little step might not do all that much. The extreme pressure on the exhaust side is also why they make "turbo cams" with much less overlap and duration. You aren't worried about the boost on the intake side going into the combustion chamber and right back out the exhaust port during overlap, but actually the exhaust coming in and filling the cylinder.