A trendline breakdown happens when price clearly falls below an upward-sloping trendline that was acting as support, indicating that buying strength is weakening and sellers are gaining control. It often signals the end of an uptrend or the start of a corrective or bearish phase. A reliable breakdown is confirmed when candles close below the trendline with strong momentum or increased volume, rather than just a temporary intraday dip. Many traders wait for a pullback or retest of the broken trendline from below before taking short positions, as this helps avoid false breakdowns and improves risk–reward management.