Basically there is a cone (funnel) in the pipe. The large end is facing the engine. Think of the exhaust as a sound wave that is traveling down the pipe. As the sound wave comes to a muffler or something else. It sets up a sound wave that is traveling back to the engine (your reversion). If the wave can go back to the engine. It robs power by slowing down the exhaust. The cone stops the wave from traveling any farther in the exhaust towards the engine. Closer to the head, better the results. I've had a set of headers that had the cones just inside the header flange at the engine. I couldn't tell the difference with them.
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