Trendline resistance is a sloping line drawn by connecting successive lower highs in a down-trending market, representing an area where selling pressure repeatedly overcomes buying interest. Each time price approaches this trendline, sellers tend to become active, causing the price to reject and move downward again, which confirms the strength of the downtrend. As long as price respects the trendline resistance, the overall trend remains bearish, while a strong breakout and close above it may signal weakening selling pressure and a potential trend reversal or trend change.