This refers to the one hour period when the market closes. During this time, spreads can spike violently and create severe issues holding positions. Usually this happens at 5PM EST. You will not be able to close positions at this time.

It is suggested you close your positions before market roll over, or make sure you have enough room with your stop losses to account for spread volatility. It is very probable that your stop loss and take profit will not be respected.
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Your daily drawdown limit will also reset at this time.