The Volume Oscillator uses the difference between two moving averages of volume to determine if the overall volume trend is increasing or decreasing.
When the Volume Oscillator rises above zero the shorter-term volume moving average has risen above the longer-term volume moving average. This means that the short-term volume trend is higher (i.e., more volume) than the longer-term volume trend. Bullish signals stem from rising prices coupled with increased volume, and falling prices coupled with decreased volume. Alternatively, if volume increases as prices fall, or volume decreases as prices rise, the market is thought to be showing signs of underlying weakness.
The logic behind this strategy is that rising prices coupled with increased volume signifies more buyers which should lead to a continued move. Similarly, falling prices coupled with increased volume (more sellers) should mean fewer buyers.