I find Spydertrader's example very important: we don't trade ES using YM as a proxy, we use YM as a leading indicator "at (ES') points of change". This also means that there'll be YM changes that won't materialize in ES changes; if for nothing else, because YM's scale is 2.5 times finer than ES'.
Quote from Spydertrader:
... I'll give you an example.
One day, while visiting Tucson, I looked at Jack and said, "What's all this nonsense about YM leading the ES? Look right here on this chart. Clearly the YM is heading in an opposite direction than the ES. As such, this YM leads the ES stuff is total rubbish."
Jack's response:
"The YM is a leading indicator of Price."
Huh? WTF does that mean??
Of course, I sat there wondering what the hell I was supposed to say next, and recall thinking at the time,
"The old man is off his rocker! I can see right there the YM doesn't lead the ES!"
A few months later, everything clicks into place, and I realize, it wasn't Jack who had the problem. I had the damn problem! I was interpreting everything Jack said through my own mental filters. I decided, on my own, that "YM leads the ES" must mean, "The YM always leads the ES" rather than, "The YM leads the ES at points of change". ...
