What year of bike are you referring to?
The reason he is telling you not to engine brake is because it can beat the crank up, especially if doing it from a high rpm. It can cause the crank to twist. The earlier tc's (like around 2002) were a bit more beefy in that department.
I think we all engine brake to a certain extent, but it's easier for the tech to tell you not to do it, instead of him taking the time to explain why.
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2015 Denim Black RG, FM ss ceramic coated 2-1-2 pipe, White Bros Original E Series Mufflers, FM ac, PV Tuner