Systems tend to run in cycles. Most traders will hop on and start trading a system after it has had a very profitable period, only to catch the next down cycle. Then, when it is in a losing period, they start tinkering with it, trying to second guess the system, instead of just sticking with it. Often they end up missing the next big profitable cycle. Following the five rules for historical testing will give you the confidence to start trading a system when it is in a drawdown.
Many traders also try to add additional filters or rules during drawdowns.
However, this breaks one of the primary rules of developing a system. Adding additional conditions or rules just makes the method less reliable in the future. Accept the drawdown as part of a necessary risk of trading. Let the system run its course and stick with it. He has developed trading systems for 20 years, is a former CPA, floor trader and member of the Chicago Board of Trade.