As explained in the above article, Bollinger band consists of 3 lines – upper, lower and middle line. The filter value is calculated by dividing the close price by lower Bollinger band and subtracting one – indicating how far the close price is from lower Bollinger band in percentage terms. Thus, a high negative value of the filter might indicate relatively lower price. Bollinger bands should always be used in tandem with other technical indicators, as in a strong downtrend prices can remain close to or below the lower Bollinger band for a long period of time.