We define Buying Volume as any volume that occurs, as an index is moving higher under the pressure of the buyers.

Generally speaking, a volume surge or volume accumulation during the price increase indicates that a large number of high-priced shares are being transferred (i.e., distributed) from one group of market participants to another. Following the surge, the number of those buyers willing to keep buying at these high (-inflated) prices becomes exhausted. Buyers are no longer willing to pay up (i.e., bid up / buy at the ask) - the market has reached a critical point where it is vulnerable to a trend reversal down.