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Backtests
A backtest shows how a rule would have done on old prices. Old prices do not include every fee, every moment you would have hesitated, or the future.
The pretty history
Rules get tuned until the past looks good. That tuning is called fit. It is also how a useless rule learns the past by heart.
What is missing
Fees, spreads, failed fills, and the days the data was wrong. A curve that never loses is a warning, not a comfort.
A backtest is a lab note. It is not a balance.
Check yourself
If a backtest never loses, should you trust it more?
No. That is a sign it was fitted to the past or ignored costs.
After this you can ask what fees were included, and be suspicious of a curve that never loses.
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