Start trading them with a small profit objective. If the trades are profitable immediately, take the profits and stand aside, and wait for another drawdown. If the trades go against you, continue trading the system until the profit objectives are reached. However, you need to have the additional capital available to stay with the system until it begins to recover from the drawdown. You also must have a high level of confidence in a system to start trading when it is in a drawdown.
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.