From Tom McClellan – Editor, The McClellan Report:
In his 1986 book, Winning On Wall Street, Martin Zweig talked about 9-1 Up Volume days. The idea is the same, since every day that qualifies as a 10-1 Up Volume Day is by definition also a 9-1 Up Volume Day. Other technicians have also done a lot of work with these types of volume signals, and 10-1 Down Volume Days are also considered to be important events.
The basic idea about why these days are important is that to get the buying and selling skewed so far in one direction requires a lot of buying or selling pressure. Generally speaking, a 10-1 Up Volume Day that arrives in an uptrend is a sign that there is still more buying to come. It is a sign of great strength.