In any market there are two major sets of participants, Retail traders (like you & I) who command relatively small amounts of capital and typically enter and exits positions quickly, and then Institutional Traders (sometimes referred to as whales) who command large amounts of capital and dictate the overall trend of the market but enter and exit positions slowly.
In this indicator we look at the distinct volume of these two sets of traders and use the net positions of this volume to determine if they are net long (Buying) in the market or net short (Selling).
When each set of traders are on opposite sides of the market (Retail are selling & Institutions are buying for example) it usually results in a battle and choppy price action... the majority of these battles are won by the Institutions as their large sums of money dictate the overall direction markets move.
Some of the best opportunities are when both sets of traders are on the same side of the market & this is where we see real momentum enter the market with quick price moves.
Happy trading =)