On-balance volume is a method used in technical analysis to detect momentum, the calculation of which relates volume to price change. OBV provides a running total of volume and shows whether this volume is flowing in or out of a given security.
OBV attempts to detect when a financial instrument is being accumulated by a large number of buyers or sold by many sellers. Traders will use an upward sloping OBV to confirm an uptrend, while a downward sloping OBV is used to confirm a downtrend. Finding a downward sloping OBV while the price of an asset is trending upward can be used to suggest that the "smart" traders are starting to exit their positions and that a shift in trend may be approaching.