In practice
Earnings and news are usually released outside regular hours, so the first reaction happens in extended trading. Traders watch pre-market price and volume to see which stocks are in play and where they are likely to open.
The pre-market high and low are commonly used as levels once the regular session begins.
What to watch for
Liquidity is thin, spreads are wide and moves can reverse the moment the main session opens. Many brokers accept only limit orders in extended hours, and stop orders may not work at all.