A trendline breakdown happens when price decisively falls below an upward (support) trendline, signaling that the prior uptrend is losing strength. It shows that buyers who were earlier supporting price are no longer in control, and selling pressure is increasing. This often marks a trend reversal or the start of a corrective/downward move, especially if the breakdown is backed by strong volume or followed by a retest where the trendline acts as resistance. Traders usually become cautious or look for short/exit opportunities after a confirmed trendline breakdown.