A trendline breakdown happens when price decisively falls below an established rising trendline, signaling that the ongoing uptrend is losing strength and sellers are gaining control. It usually indicates a potential shift in market sentiment from bullish to bearish, especially when accompanied by strong volume or a clear closing below the trendline. After the breakdown, the broken trendline often acts as a new resistance zone, where price may retest before continuing lower. Traders commonly use this signal to exit long positions or look for short opportunities, while confirming it with other indicators like volume, support levels, or candlestick patterns for better reliability.