A trendline breakdown occurs when price decisively falls below an upward sloping trendline that was previously acting as dynamic support. In an uptrend, the trendline connects higher lows and reflects buying strength; when price closes below it with strong volume, it signals that buyers are losing control and sellers are gaining momentum. This breakdown often indicates a potential trend reversal or the start of a corrective phase. Traders usually wait for confirmation such as a strong bearish candle or a retest of the broken trendline (now acting as resistance) before entering short positions to avoid false breakdowns.