I have noticed that when comparing results of different EMA pairs and building heatcharts the main thing that matters is when the sell signal is generated. The pairs which generate sell signal at the bottom of the panic sell pit make less profit then the pairs which generate a sell signal later when the price bounces back up a little, or even when the price stabilizes after the fall.
The point where the sell signal is generated, while the bubble collapse, seems to be the most influencing factor on the result of an EMA-cross systems comparison.
Before answering your below questions, I feel the need to suggest that you may be curve-fitting. If you are to a point where you are trying to find a system which catches precise market action, you are too far "in the weeds". The market rarely repeats itself exactly so a system which is optimized around a certain market event will probably not perform well in the future. Think general thoughts - a moving average crossover system is designed to catch an average trend and hold it through its life-cycle. A system designed to catch a specific trend will probably not hold up in the future.
Adding some common sense while closing long positions during the bubble crash can increase the profit greatly. I think that somewhere on this thread you told that if you see a flashcrash you shouldn't sit around waiting fir the lines to cross. But doing this means ruining the whole method as you can no longer expect that the system used in a way it was not optimised for will bring you to the point of it's statistical expectation.
I mean that optimising the system by 2 parameters and then making decigions upon the change of some third parameter or a gut feeling makes no sense.
You are correct - if you do not trade the system exactly as tested, your returns will be materially different. Incorporating "gut" into your strategy makes very little sense in that it essentially invalidates your backtest results. I highly discourage gut decisions.
And the third is adding a third parameter for optimisation like a trailing stop loss which can imitate a manual position closing.
A stop-loss is a key component of trading. It is dangerous to trade without a stop-loss. Over the long run, you will probably encounter a trade which will completely destroy your account if you do not set stops. I suggest you research this more and incorporate a stop-loss into your trading system.
And the second question is about re-optimising the system. Somewhere on this thread you said that the 21-10 system is not the best one according to backtesting any more, but as far as I could understand you are against re-optimising it and choosing another EMA pair due to overfitting. Did I get it correctly?
Here's the gist of what I was saying - optimization only works up to a point. If you continuously optimize your trading strategy, you will constantly be chasing different variables. The markets are always changing and a system that is heavily reliant on a specific set of inputs probably won't be able to keep up. 10/21 might not be the best optimized solution, but I am confident that the indicators have longevity in that they catch the market action that I am after and have performed well in several markets for decades.