This screen finds stocks whose price has just bounced down from the middle Bollinger Band, the 20-day moving average. Bollinger Bands(20,2), created by John Bollinger, plot a 20-day simple moving average as the middle band and upper and lower bands two standard deviations away, so the bands widen when volatility rises and narrow when it falls. A rejection at the 20-day average is a classic trend-continuation sell setup in a downtrend.
