The more volatile the security being analyzed becomes, the wider the range becomes between the two outer lines or bands. Similar to moving averages, Bollinger Bands can be used to create trading strategies that can be easily computerized and tested. A common use is as a contrarian strategy, in which the investor sells when a security price reaches the upper band and buys when it reaches the lower band. This strategy assumes that the security price will stay within the bands.
A contrarian investor acts contrary to the market trend. When the prices of security reach the upper band of the Bollinger bands, a contrarian sells the security, and he buys the security when the price of the security reaches the lower band of the Bollinger bands.