Traders read the tape - ticker tape in the old days to determine the recent buying and selling pressure. Here lies the importance of volume. It's easy to spot a large buyer or seller when the volume is light. Big buyers usually enter before big moves.
Jesse Livermore read the tape and never fought it.
Of course there is always a reason for fluctuations, but the tape does not concern itself with the why or wherefore. It doesn’t go into explanations. I didn’t ask the tape why when I was fourteen, and I don’t ask it today at forty…, Reminiscences of a Stock Operator.
While the tape, often triggered by price in reaction or anticipation of the new flow, is triggered by price, it's fueled by force. Volume is the domain of force. Traders/investors must learn to read the force behind the trend. You can see this in the example from the SPX.
Volume, an exceptionally important trend followed by my computers, has been contracting steadily after the 2008 crisis. This suggests the following: (1) money is getting off the grid and generally afraid - the latter is bullish, (2) the public is not playing in the stock rally - also bullish, and (3) volume will likely expand as volatility rises - a difficult combination for non-professionals that could easily mirror the 1927 to 1929 cycle inversion.