I just had a long talk with one of the engineers (Jeff) at NGK about the BR9ECS vs BR9ES plugs. He told me that the difference is that the BR9ECS plug has a low angle ground electrode. (that's it...that's all...no special metals, no different depth, not hotter, not colder etc etc)
That being said, that causes a difference in the flame front in the combustion chamber. The ECS plug with the low angle ground electrode causes a more controlled and equal flame extension into the chamber where the ES plug causes a 3/4 circle front (blocked by the electrode).
In some engines this makes a difference in detonation control at higher speeds and loads. He concluded that the engine manufactor must have experienced some problems under testing and recommended the ECS plug to provide the best performance and reliability.
He would not recommend using the ES plug instead of the ECS plug due to the likelihood of detonation problems under high loads &/or rpm, but said that the ES plug might work just fine in some engines until conditions in the combustion chamber got hotter and more likely to start detonation.
Been talked about before. Plugs get hot, electrodes melt and start to glow causing pre ignition. Its like having a piece of carbon stuck to a valve in a 4 stroke you have pre detenation. Burns out the plug and they foul.