The J line is a measure of divergence between %D and %K. The value of J can go beyond [0, 100] range. Values of J line over 100 or under 0 are supposedly the most bullish or bearish for larger swings.

One needs to watch as the %D is in overbought and oversold areas (20 - 80). As long as the J line is beyond 0 and 100 percent and has not hooked up or down the price is going to remain in overbought or oversold area. One needs to consider to sell the overbought or buy the oversold when %D is above 80 or below 20 and J line begins to hook down or up. Other indicators such as momentum, volume, support and resistance, etc. need to be considered for confirmation.