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It's when you make the openings bigger. More flow. Like in my case, I have this BBK 70MM T/B comming. If I were to just install it, I'd lose power. So I have to port the opening to meet the 70mm T/B.
Yes. You are either making them bigger, or changing the shape slightly. Porting is making the openings bigger. Now if you really want to get into it, you port AND polish! More flow, and less turbulance.