Recently, someone asked me how to use the KDJ indicator again, so I decided to write something to clarify it for everyone. To be honest, this indicator is indeed very popular among retail traders, but many people only know how to look at the lines and don't understand the underlying logic, resulting in frequent losses.

Let me start with a real case. In February 2016, the Hong Kong Hang Seng Index was falling all the way down, with prices making lower lows and looking utterly hopeless. But savvy traders noticed something? The KDJ indicator's trend was making higher lows with each wave. This is called a bullish divergence at the bottom, a classic reversal signal. As a result, on February 19th, the Hang Seng Index shot up with a large bullish candle of 965 points, a 5.27% increase. Those who saw this signal early on managed to buy at the bottom.

See, this is the power of the KDJ indicator. It can help you discover opportunities hidden within the trend while others are still panicking.

So what exactly is the KDJ indicator? Simply put, it is a stochastic indicator that measures the position of the closing price within a certain period's price range to determine if the market is overbought or oversold. The indicator has three lines: K (fast line), D (slow line), and J. The K and D lines are used to judge overbought and oversold conditions, while the J line reflects the deviation between K and D.

Regarding the formula of the KDJ indicator, you don't need to memorize it by heart. The core is to first calculate the raw stochastic value RSV, which is (today's closing price - lowest price over n days) ÷ (highest price over n days - lowest price over n days) × 100. Then, based on this RSV, you use smoothing formulas to calculate the K, D, and J values. Most trading platforms do this calculation for you; you just need to set the parameters, usually (9,3,3), and you'll see the trend chart calculated by the KDJ formula.

How to use it in practice? There are several key signals worth paying attention to.

First is overbought and oversold. Draw lines at 80 and 20 on the chart; when the K and D lines exceed 80, it indicates an overbought zone, and when they drop below 20, it indicates an oversold zone. The J line is more sensitive; exceeding 100 is overbought, below 10 is oversold.

Second is the golden cross and death cross. When K and D are both below 20, and K crosses above D, it's called a low-level golden cross, a buy signal. Conversely, when K and D are above 80, and K crosses below D, it's called a high-level death cross, a sell signal. For example, in the Hang Seng case, a low-level golden cross appeared on February 26th, prompting investors to increase their positions, and the next day, the index rose by 4.20%. On April 29th, a high-level death cross appeared, leading them to exit and lock in profits.

Third is divergence. When the price makes higher peaks but the KDJ values make lower peaks, it's called a top divergence, a sell signal. Conversely, if the price makes lower peaks but the KDJ peaks are higher, it's called a bottom divergence, a buy signal. This signal is often the most powerful.

There are also double bottoms (W-shaped) and double tops (M-shaped). When KDJ forms a W or triple bottom below 50, it indicates an imminent reversal upward. When it forms an M or triple top above 80, it suggests a reversal downward. The more bottoms or tops, the larger the potential move.

But I must be honest, the KDJ indicator also has pitfalls. The first is lagging. In very strong or very weak markets, it can give early signals that lead you to buy or sell prematurely, resulting in frequent stop-losses. The second is delay; since it is based on past price data, it may react too slowly during rapid market changes. The third is false signals; especially during sideways consolidation, it can produce many fake buy or sell points.

Therefore, never rely solely on the KDJ indicator. My advice is to combine it with other indicators, such as candlestick patterns, volume, moving averages, etc., to improve accuracy. During trading, accumulate experience over time, and you'll gradually learn when signals are reliable and when they might be traps.

Finally, a tip: if you want to practice, open a demo account on a trading platform. No real money needed. Practice using the KDJ indicator in real market conditions. Watch the chart's real-time changes, and make trading decisions based on the calculated results of the KDJ formula. This way, you'll learn fastest. Remember, no technical indicator is perfect; the key is to understand each tool's personality and use it flexibly in actual trading.