Quick google search - boost pressure is diverted around the engine (instead of going through it) into the exhaust manifold. It works by delaying the ignition timing very slightly to balance an inherent loss in combustion efficiency with increased pressure at the charging side of the turbo. This is achieved as a small amount of fuel/air mixture escapes through the exhaust valves and combusts in the hot exhaust manifold spooling the turbocharger creating higher usable pressure. Turbochargers are manufactured to extremely fine tolerances and not designed for explosive pressure waves.
Ok, sill not grasping how the valves take the hit. Sounds like the turbo is getting pretty worked just as I stated. Not trying to argue a point but am genuinely curious of the process. Is there something that I'm not taking into account?
Apologies for the thread jack but to answer your Q on the midpipe The material wont make much difference in tone or loudness from the midpipe. The benefit here is weight savings (a few lbs) What matters is if you want resonated or non resonated, Catted or no cats, Diameter plays a role. If 90mm, most mids are bolt connection, if 4" most mids are vband. Choose what fits your application.