A trending market should see more participants and increased levels of volume. Increasing price and decreasing volume might suggest a lack of interest, and this is a warning of a potential reversal. This can be hard to wrap your mind around, but the simple fact is that a price drop (or rise) on little volume is not a strong signal.
In a rising or falling market, we can see exhaustion moves. These are generally sharp moves in price combined with a sharp increase in volume, which signals the potential end of a trend. Participants who waited and are afraid of missing more of the move pile in at market tops/bottoms, exhausting the number of buyers/sellers. Once these have exhausted the move then market usually reverses to look for more liquidity.
Volume can be useful in identifying bullish signs. For example, imagine volume increases on a price decline and then the price moves higher, followed by a move back lower. If the price on the move back lower doesn’t fall below the previous low, and volume is diminished on the second decline, then this is usually interpreted as a bullish sign.
After a long price move higher or lower, if the price begins to range with little price movement and heavy volume, this might indicate that a reversal is underway, and prices will change direction.
On the initial breakout from a range or other chart pattern, a rise in volume indicates strength in the move. Little change in volume or declining volume on a breakout indicates a lack of interest and a higher probability for a false breakout.