A trendline resistance breakout occurs when the price, which has been forming lower highs under a downward-sloping resistance trendline, breaks and closes decisively above that trendline with strong buying momentum. This signals that sellers are losing control and buyers are stepping in, potentially leading to a trend reversal or the start of a new uptrend. The breakout becomes more reliable if it is supported by higher volume and followed by a successful retest of the broken trendline as new support, increasing the probability of further bullish movement.