Most rules only work in one regime
A moving-average crossover earns its keep in trends and bleeds in ranges. Mean-reversion does the opposite. Defining market conditions is how you stop a strategy from trading in the environment it was never designed for.
Practical filters
Common condition filters include a long-term trend filter (price above or below a 200-period average), a volatility band (ATR within a range), and a participation filter (volume above a baseline).
Each filter you add reduces the number of trades. Fewer, better-qualified trades usually beat more marginal ones once costs are included.
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Run the same rules with and without the condition filter over five years. If the filter only improves results in one narrow window, it is probably curve fitting rather than a real regime effect.