Intermarket analysis trades one market from the behaviour of another: the Australian dollar from copper or gold, the yen from equities, a currency from the dollar index, oil from the Canadian dollar. The relationships are real and widely discussed.
The claim is that the leading market moves first, and the lagging one can be traded on it.
We tested intermarket rules, using one market's recent move to trade another, across 17 years of data with real trading costs, and found no edge. The correlations existed, and they were simultaneous. By the time one market had moved, the other had moved with it, and there was no lag to trade.
The reason is that these relationships are known to every participant, and known relationships are priced at the same moment on both sides.
If you trade it anyway, measure the lead time between the two markets with real data before assuming one leads.