Regardless of the market space (energy, currencies, grains, metals, interest rates, equities) or the holding timeframe (months, weeks, days, minutes, high frequency stat arb) the basic tenets for relative value spread trading remains: capture the difference between the products either through selling convergence (narrowing spread) or buying divergence (widening spread).
The trader's edge in spread trading is derived from: 1. the combination of instruments the trader chooses to trade, and 2. the statistical modeling or technical analysis the trader applies to the spread differential.