Resets at the start of the day's session (9:50 am on the ASX).
Takes the first candle and checks if its a green candle (close > open), and adds the volume to a total (which starts at 0 at the start of the day). Subtracts it if the candles in red.
Continues to do this along with all the candles.
The volume is also multiplied by the difference between open and close so that large candles with large volume have more weight and move the indicator more.