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Well, I researched this a bit and there is sound theory behind this thing.
Ehxaust throttling have been shown to increase FE and reduce emissions at low rpm / low loads by increasing internal EGR, that is having an increased residual gas fraction staying in the cylinder taking up space that would otherwise being filled by fresh air. The residual gas fraction being gas products of late combustion, they contains the most NOx and unburnt HC.
Exhaust throttling also increase the pressure and temps in the exhaust system. This could probably also help getting your cat to efficient operating temp more quickly.
The downside is the increased risk of knock as the residual gas fraction increase.
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