During that period price fell well below the open, then buyers pushed it back up near the high.
Note: The four chart values show a low below the open and a close near the high. They do not establish who caused the move.
Rule of thumb
Many traders look for a lower shadow of about twice the body or more.
This is a common rule of thumb, not a law.
Context
A hammer is usually discussed after a run of falling candles.
The body can be green or red.
The same shape after a rise is called a hanging man.
How traders use it
Traders may examine whether a subsequent candle closes above the hammer and may define conditions that would invalidate an interpretation.
These are descriptions of analytical conventions, not instructions or recommendations to trade.
A close below the hammer's low may be considered evidence against a proposed bullish interpretation.
Both continuation and reversal can occur.
What it is not
A hammer alone is not a buy signal.
Research
Marshall, Young & Rose (2007), “Market Timing with Candlestick Technical Analysis,” Journal of Financial Transformation, 20, 18–25.
The research examined candlestick strategies in a defined equity-market setting and did not find overall profitability for the tested strategies.
This conclusion concerns the tested strategies generally, not the Hammer pattern individually.