Bollinger Bands consist of a middle line (usually a 20-period SMA) and an upper/lower band plotted a chosen number of standard deviations (commonly 2) away from it. Because the bands are based on standard deviation, they widen automatically in volatile conditions and tighten in quiet ones. The band width itself is a volatility read.
A close outside the bands is a statistically unusual move, the basis for two opposite trading styles: reversion (buy a close below the lower band, betting on a snap back to the middle) or breakout (buy a close above the upper band after a tight "squeeze," betting the contraction is resolving into a new expansion).
zengtrade uses Bollinger Bands both ways: a reversion strategy that buys the lower-band dip inside an uptrend, and, among the strategies not yet featured to users, a squeeze-breakout variant that does the opposite.
Educational content, not investment advice. zengtrade is paper-first and non-custodial.