The US market opens at 6:30 AM PST. In the 30 minutes before the market open, BTC and the SPY/QQQ futures tend to be highly correlated, but let's say NQ (which is a futures contract that tracks the Nasdaq) rises .5% but BTC only rises .25%, then this would be relative weakness on the part of BTC. The trade would be to short BTC and go long QQQ in equal size.