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more air getting in than the maf sees. ecu sees the result in the exhaust and tries to add more fuel to compensate. this is because the ecu wants, at idle and cruise, for the mixture to be 14.7 parts air to 1 part fuel (the 'air fuel mixture' or afr). once the ecu adds about 20% or so more fuel and still can't hit 14.7:1 afr then you get the code.
when hooning around the ecu tries to add a bit more gas to the ratio (ie, 12.5 parts air to 1 part fuel) in order to keep things cool. the ecu defines 'hooning' by throttle position (ie, 'wide open throttle' or wot) rpm, engine temperature. this 12.5:1 afr makes less power but helps keep the engine alive. unfortunately, the ecu doesn't have any sensors on the exhaust for this richer mixture (the 'narrow band' oxygen sensor used by the ecu can only detect an afr of14.7:1) so the additional air sneaking past the maf goes undetected, leans out the AFR, and you get more power. and your engine runs hotter.
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