While running your optimizer, I'm sure you saw some results that clearly lost a lot of money. You might even get frustrated. But not so fast. You see, if those trades have large TP and SL targets, then doing the exact opposite can make you money.
Noticed that I said large TP and SL levels. When your TP's and SL's are large, the fees that you pay to your broker are less significant. So if your trade was scalping and it was losing money, then trading in the opposite direction could also cause you to lose money as well.
So I know that this may seem obvious, but the first few weeks that I got started, I never really paid attention to them. Now I learn from them.