We define Selling Volume as any volume that occurs, as an index is moving lower.

Generally speaking, larger than average amounts of volume to the downside indicate Selling Volume surge. Selling volume surge and selling volume accumulation indicates that a large number of low-priced shares are being transferred (i.e., distributed) from one group of market participants to another. Big volume surge during the price decline means that the price dropped to the level when it become attractive to many buyers who started to buy low priced shares in big amounts from panic sellers. Following the surge, the number of those sellers that are still willing to "keep giving away" shares at low ("bargain") prices becomes exhausted. Sellers are no longer willing to dump their shares at the bid - the market has reached a critical point where it is vulnerable to a trend reversal up. (plus here buyers move in who are bottom fishing and shorts start to cover).