A hammer candle at the bottom of a sell-off indicates that a base is forming and buyers are pushing the price back up; the next candle is likely to open higher, signaling the first candle of a bullish reversal.
"So, a hammer in the context of a sell-off... That communicates a little bit more strength. So what the hammer is telling us is that we're basing out that we sold off, we rallied back up, and the next candle is going to open more or less at the high, right? And that is going to be your first candle of reversal."
