I wasn't questioning the benefit of engine braking, I only asked out of curiosity as to what was actually going on in the engine. As we know, trucks fitted with the old type exhaust brakes worked by the compression of the exhaust being released to the inlet on valve overlap at the end of the exhaust stroke. The compression stroke had no effect as after TDC the resultant cylinder pressure acts pushing the piston down. Other variants (ie Jacobs brakes etc) change the valve timing, effectively turning the engine into compressor. If the AT only works by closing the inlet butterfly, I could be wrong, but if this is the case, it seems little more than a token gesture.
I am sure there is a Honda technician out there that will answer my curiosity.