A trendline acts as resistance when it is drawn by connecting lower highs in a downtrend, showing the area where sellers repeatedly enter the market. Each time price rises toward this descending line, selling pressure often increases, causing the price to fall again. This happens because traders view the trendline as a key level where the existing downtrend may continue. If price gets rejected multiple times from the trendline, it strengthens its importance, while a strong breakout above it may signal weakening bearish momentum and a possible trend reversal.