"What's the McClellan Oscillator?" My understanding of this site's explanation is that movements in the share prices of a few large companies, heavily weighted in a stock market index, can mask what is going on in the market generally. And when those large companies quieten down, investors may notice an opposite trend has been developing, and they'll pile in after it.
For example, if shares in major banks have been crashing, but other companies have been rising, the market as a whole may drift down, but then...
Signs and portents, signs and portents.