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Phil's referring to trimming out the engine after hopping up on plane.

At displacement speeds, the engine is trimmed down all the way. But after accelerating and transitioning to plane, the engine needs to be progressively trimmed out/up as speed increases. This improves the propeller's angle of attack and reduces drag (and steering effort), resulting in higher engine rpm and boat speed.

If you are not trimming out the engine, you are leaving a lot of performance on the table. You are also likely exerting a great deal of effort to steer the boat while on plane.