A hammer pattern at trendline support is a strong bullish reversal signal that appears when price tests an upward-sloping trendline support and rejects lower levels. The hammer has a small real body near the top and a long lower shadow, showing that sellers pushed price down but buyers regained control before the close. When this pattern forms exactly at a trendline support zone, it indicates strong buying interest and demand absorption, often leading to a bounce or continuation of the uptrend. Confirmation with the next bullish candle and higher volume further increases the reliability of this setup.