A market in an uptrend will continually make higher highs and higher lows. A market in a downtrend will continually make lower highs and lower lows. When you see these conditions end you know the trend is also coming to an end. So in an uptrend if price fails to make a new higher high or if it drops below the last low and makes a lower low you know the trend is coming to an end and that it will soon reverse.