Price tells you where the market is. Momentum tells you how hard it is pushing to get there. A move can keep climbing while the force behind it quietly fades, and if you can see that fading, you get an early warning long before the chart rolls over.
The most common momentum tool is the RSI, short for Relative Strength Index. It is a line that runs from zero to one hundred and measures how strong recent up moves are compared to recent down moves. Above seventy is often called overbought, meaning the buying has been intense. Below thirty is often called oversold, meaning the selling has been intense. On its own that is a rough gauge, not a signal to trade.
The real power of the RSI is divergence. Divergence is when price and the RSI disagree. Bearish divergence is when price makes a higher high but the RSI makes a lower high. The chart looks strong, but the engine behind it is weaker than last time. Each new peak is being made with less force, which often comes right before a top.
Bullish divergence is the mirror and shows up at bottoms. Price carves a lower low, but the RSI makes a higher low. Sellers pushed price to a fresh low, yet with less power than before. The selling is running out of steam even as the price still looks ugly, and that is often where a bounce begins.
Momentum measures the force behind a move, and the RSI is the common way to read it. Divergence is when price and the RSI disagree: a higher high in price on a lower high in RSI warns of a top, a lower low in price on a higher low in RSI hints at a bottom.
Tip. Divergence is a warning, not a trigger. It can drag on far longer than feels reasonable, so wait for price itself to confirm, like a break of structure, before you act on it.